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787 lines
20 KiB
Markdown
787 lines
20 KiB
Markdown
---
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comments: true
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---
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# 堆
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「堆 Heap」是一棵限定条件下的「完全二叉树」。根据成立条件,堆主要分为两种类型:
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- 「大顶堆 Max Heap」,任意结点的值 $\geq$ 其子结点的值;
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- 「小顶堆 Min Heap」,任意结点的值 $\leq$ 其子结点的值;
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![min_heap_and_max_heap](heap.assets/min_heap_and_max_heap.png)
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## 堆术语与性质
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- 由于堆是完全二叉树,因此最底层结点靠左填充,其它层结点皆被填满。
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- 二叉树中的根结点对应「堆顶」,底层最靠右结点对应「堆底」。
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- 对于大顶堆 / 小顶堆,其堆顶元素(即根结点)的值最大 / 最小。
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## 堆常用操作
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值得说明的是,多数编程语言提供的是「优先队列 Priority Queue」,其是一种抽象数据结构,**定义为具有出队优先级的队列**。
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而恰好,**堆的定义与优先队列的操作逻辑完全吻合**,大顶堆就是一个元素从大到小出队的优先队列。从使用角度看,我们可以将「优先队列」和「堆」理解为等价的数据结构。因此,本文与代码对两者不做特别区分,统一使用「堆」来命名。
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堆的常用操作见下表(方法命名以 Java 为例)。
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<p align="center"> Table. 堆的常用操作 </p>
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<div class="center-table" markdown>
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| 方法 | 描述 | 时间复杂度 |
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| --------- | -------------------------------------------- | ----------- |
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| add() | 元素入堆 | $O(\log n)$ |
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| poll() | 堆顶元素出堆 | $O(\log n)$ |
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| peek() | 访问堆顶元素(大 / 小顶堆分别为最大 / 小值) | $O(1)$ |
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| size() | 获取堆的元素数量 | $O(1)$ |
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| isEmpty() | 判断堆是否为空 | $O(1)$ |
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</div>
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我们可以直接使用编程语言提供的堆类(或优先队列类)。
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!!! tip
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类似于排序中“从小到大排列”和“从大到小排列”,“大顶堆”和“小顶堆”可仅通过修改 Comparator 来互相转换。
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=== "Java"
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```java title="heap.java"
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/* 初始化堆 */
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// 初始化小顶堆
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Queue<Integer> minHeap = new PriorityQueue<>();
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// 初始化大顶堆(使用 lambda 表达式修改 Comparator 即可)
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Queue<Integer> maxHeap = new PriorityQueue<>((a, b) -> { return b - a; });
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/* 元素入堆 */
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maxHeap.add(1);
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maxHeap.add(3);
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maxHeap.add(2);
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maxHeap.add(5);
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maxHeap.add(4);
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/* 获取堆顶元素 */
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int peek = maxHeap.peek(); // 5
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/* 堆顶元素出堆 */
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// 出堆元素会形成一个从大到小的序列
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peek = heap.poll(); // 5
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peek = heap.poll(); // 4
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peek = heap.poll(); // 3
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peek = heap.poll(); // 2
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peek = heap.poll(); // 1
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/* 获取堆大小 */
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int size = maxHeap.size();
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/* 判断堆是否为空 */
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boolean isEmpty = maxHeap.isEmpty();
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/* 输入列表并建堆 */
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minHeap = new PriorityQueue<>(Arrays.asList(1, 3, 2, 5, 4));
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```
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=== "C++"
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```cpp title="heap.cpp"
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/* 初始化堆 */
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// 初始化小顶堆
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priority_queue<int, vector<int>, greater<int>> minHeap;
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// 初始化大顶堆
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priority_queue<int, vector<int>, less<int>> maxHeap;
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/* 元素入堆 */
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maxHeap.push(1);
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maxHeap.push(3);
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maxHeap.push(2);
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maxHeap.push(5);
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maxHeap.push(4);
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/* 获取堆顶元素 */
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int peek = maxHeap.top(); // 5
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/* 堆顶元素出堆 */
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// 出堆元素会形成一个从大到小的序列
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maxHeap.pop(); // 5
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maxHeap.pop(); // 4
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maxHeap.pop(); // 3
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maxHeap.pop(); // 2
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maxHeap.pop(); // 1
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/* 获取堆大小 */
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int size = maxHeap.size();
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/* 判断堆是否为空 */
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bool isEmpty = maxHeap.empty();
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/* 输入列表并建堆 */
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vector<int> input{1, 3, 2, 5, 4};
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priority_queue<int, vector<int>, greater<int>> minHeap(input.begin(), input.end());
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```
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=== "Python"
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```python title="heap.py"
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```
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=== "Go"
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```go title="heap.go"
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// Go 语言中可以通过实现 heap.Interface 来构建整数大顶堆
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// 实现 heap.Interface 需要同时实现 sort.Interface
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type intHeap []any
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// Push heap.Interface 的方法,实现推入元素到堆
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func (h *intHeap) Push(x any) {
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// Push 和 Pop 使用 pointer receiver 作为参数
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// 因为它们不仅会对切片的内容进行调整,还会修改切片的长度。
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*h = append(*h, x.(int))
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}
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// Pop heap.Interface 的方法,实现弹出堆顶元素
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func (h *intHeap) Pop() any {
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// 待出堆元素存放在最后
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last := (*h)[len(*h)-1]
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*h = (*h)[:len(*h)-1]
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return last
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}
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// Len sort.Interface 的方法
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func (h *intHeap) Len() int {
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return len(*h)
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}
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// Less sort.Interface 的方法
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func (h *intHeap) Less(i, j int) bool {
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// 如果实现小顶堆,则需要调整为小于号
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return (*h)[i].(int) > (*h)[j].(int)
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}
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// Swap sort.Interface 的方法
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func (h *intHeap) Swap(i, j int) {
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(*h)[i], (*h)[j] = (*h)[j], (*h)[i]
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}
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// Top 获取堆顶元素
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func (h *intHeap) Top() any {
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return (*h)[0]
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}
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/* Driver Code */
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func TestHeap(t *testing.T) {
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/* 初始化堆 */
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// 初始化大顶堆
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maxHeap := &intHeap{}
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heap.Init(maxHeap)
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/* 元素入堆 */
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// 调用 heap.Interface 的方法,来添加元素
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heap.Push(maxHeap, 1)
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heap.Push(maxHeap, 3)
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heap.Push(maxHeap, 2)
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heap.Push(maxHeap, 4)
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heap.Push(maxHeap, 5)
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/* 获取堆顶元素 */
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top := maxHeap.Top()
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fmt.Printf("堆顶元素为 %d\n", top)
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/* 堆顶元素出堆 */
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// 调用 heap.Interface 的方法,来移除元素
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heap.Pop(maxHeap)
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heap.Pop(maxHeap)
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heap.Pop(maxHeap)
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heap.Pop(maxHeap)
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heap.Pop(maxHeap)
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/* 获取堆大小 */
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size := len(*maxHeap)
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fmt.Printf("堆元素数量为 %d\n", size)
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/* 判断堆是否为空 */
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isEmpty := len(*maxHeap) == 0
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fmt.Printf("堆是否为空 %t\n", isEmpty)
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}
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```
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=== "JavaScript"
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```javascript title="heap.js"
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// JavaScript 未提供内置 heap 类
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```
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=== "TypeScript"
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```typescript title="heap.ts"
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// TypeScript 未提供内置堆 Heap 类
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```
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=== "C"
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```c title="heap.c"
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```
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=== "C#"
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```csharp title="heap.cs"
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/* 初始化堆 */
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// 初始化小顶堆
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PriorityQueue<int, int> minHeap = new PriorityQueue<int, int>();
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// 初始化大顶堆(使用 lambda 表达式修改 Comparator 即可)
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PriorityQueue<int, int> maxHeap = new PriorityQueue<int, int>(Comparer<int>.Create((x, y) => y - x));
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/* 元素入堆 */
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maxHeap.Enqueue(1, 1);
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maxHeap.Enqueue(3, 3);
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maxHeap.Enqueue(2, 2);
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maxHeap.Enqueue(5, 5);
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maxHeap.Enqueue(4, 4);
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/* 获取堆顶元素 */
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int peek = maxHeap.Peek();//5
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/* 堆顶元素出堆 */
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// 出堆元素会形成一个从大到小的序列
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peek = maxHeap.Dequeue(); // 5
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peek = maxHeap.Dequeue(); // 4
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peek = maxHeap.Dequeue(); // 3
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peek = maxHeap.Dequeue(); // 2
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peek = maxHeap.Dequeue(); // 1
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/* 获取堆大小 */
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int size = maxHeap.Count;
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/* 判断堆是否为空 */
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bool isEmpty = maxHeap.Count == 0;
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/* 输入列表并建堆 */
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minHeap = new PriorityQueue<int, int>(new List<(int, int)> { (1, 1), (3, 3), (2, 2), (5, 5), (4, 4), });
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```
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=== "Swift"
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```swift title="heap.swift"
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// Swift 未提供内置 heap 类
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```
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=== "Zig"
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```zig title="heap.zig"
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```
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## 堆的实现
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下文实现的是「大顶堆」,若想转换为「小顶堆」,将所有大小逻辑判断取逆(例如将 $\geq$ 替换为 $\leq$ )即可,有兴趣的同学可自行实现。
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### 堆的存储与表示
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在二叉树章节我们学过,「完全二叉树」非常适合使用「数组」来表示,而堆恰好是一棵完全二叉树,**因而我们采用「数组」来存储「堆」**。
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**二叉树指针**。使用数组表示二叉树时,元素代表结点值,索引代表结点在二叉树中的位置,**而结点指针通过索引映射公式来实现**。
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具体地,给定索引 $i$ ,那么其左子结点索引为 $2i + 1$ 、右子结点索引为 $2i + 2$ 、父结点索引为 $(i - 1) / 2$ (向下整除)。当索引越界时,代表空结点或结点不存在。
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![representation_of_heap](heap.assets/representation_of_heap.png)
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我们将索引映射公式封装成函数,以便后续使用。
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=== "Java"
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```java title="my_heap.java"
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[class]{MaxHeap}-[func]{left}
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[class]{MaxHeap}-[func]{right}
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[class]{MaxHeap}-[func]{parent}
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```
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=== "C++"
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```cpp title="my_heap.cpp"
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[class]{MaxHeap}-[func]{left}
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[class]{MaxHeap}-[func]{right}
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[class]{MaxHeap}-[func]{parent}
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```
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=== "Python"
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```python title="my_heap.py"
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```
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=== "Go"
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```go title="my_heap.go"
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[class]{maxHeap}-[func]{left}
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[class]{maxHeap}-[func]{right}
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[class]{maxHeap}-[func]{parent}
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```
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=== "JavaScript"
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```javascript title="my_heap.js"
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[class]{MaxHeap}-[func]{#left}
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[class]{MaxHeap}-[func]{#right}
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[class]{MaxHeap}-[func]{#parent}
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```
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=== "TypeScript"
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```typescript title="my_heap.ts"
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[class]{MaxHeap}-[func]{left}
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[class]{MaxHeap}-[func]{right}
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[class]{MaxHeap}-[func]{parent}
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```
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=== "C"
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```c title="my_heap.c"
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[class]{maxHeap}-[func]{left}
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[class]{maxHeap}-[func]{right}
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[class]{maxHeap}-[func]{parent}
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```
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=== "C#"
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```csharp title="my_heap.cs"
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[class]{MaxHeap}-[func]{left}
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[class]{MaxHeap}-[func]{right}
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[class]{MaxHeap}-[func]{parent}
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```
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=== "Swift"
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```swift title="my_heap.swift"
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[class]{MaxHeap}-[func]{left}
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[class]{MaxHeap}-[func]{right}
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[class]{MaxHeap}-[func]{parent}
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```
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=== "Zig"
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```zig title="my_heap.zig"
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[class]{MaxHeap}-[func]{left}
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[class]{MaxHeap}-[func]{right}
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[class]{MaxHeap}-[func]{parent}
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```
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### 访问堆顶元素
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堆顶元素是二叉树的根结点,即列表首元素。
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=== "Java"
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```java title="my_heap.java"
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[class]{MaxHeap}-[func]{peek}
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```
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=== "C++"
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```cpp title="my_heap.cpp"
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[class]{MaxHeap}-[func]{peek}
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```
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=== "Python"
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```python title="my_heap.py"
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```
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=== "Go"
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```go title="my_heap.go"
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[class]{maxHeap}-[func]{peek}
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```
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=== "JavaScript"
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||
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```javascript title="my_heap.js"
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[class]{MaxHeap}-[func]{peek}
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```
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=== "TypeScript"
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```typescript title="my_heap.ts"
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[class]{MaxHeap}-[func]{peek}
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```
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=== "C"
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||
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```c title="my_heap.c"
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[class]{maxHeap}-[func]{peek}
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```
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=== "C#"
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||
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||
```csharp title="my_heap.cs"
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[class]{MaxHeap}-[func]{peek}
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```
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||
|
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=== "Swift"
|
||
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```swift title="my_heap.swift"
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[class]{MaxHeap}-[func]{peek}
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```
|
||
|
||
=== "Zig"
|
||
|
||
```zig title="my_heap.zig"
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[class]{MaxHeap}-[func]{peek}
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```
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### 元素入堆
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给定元素 `val` ,我们先将其添加到堆底。添加后,由于 `val` 可能大于堆中其它元素,此时堆的成立条件可能已经被破坏,**因此需要修复从插入结点到根结点这条路径上的各个结点**,该操作被称为「堆化 Heapify」。
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||
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考虑从入堆结点开始,**从底至顶执行堆化**。具体地,比较插入结点与其父结点的值,若插入结点更大则将它们交换;并循环以上操作,从底至顶地修复堆中的各个结点;直至越过根结点时结束,或当遇到无需交换的结点时提前结束。
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=== "<1>"
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![heap_push_step1](heap.assets/heap_push_step1.png)
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=== "<2>"
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![heap_push_step2](heap.assets/heap_push_step2.png)
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=== "<3>"
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![heap_push_step3](heap.assets/heap_push_step3.png)
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=== "<4>"
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![heap_push_step4](heap.assets/heap_push_step4.png)
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=== "<5>"
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![heap_push_step5](heap.assets/heap_push_step5.png)
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=== "<6>"
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![heap_push_step6](heap.assets/heap_push_step6.png)
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设结点总数为 $n$ ,则树的高度为 $O(\log n)$ ,易得堆化操作的循环轮数最多为 $O(\log n)$ ,**因而元素入堆操作的时间复杂度为 $O(\log n)$** 。
|
||
|
||
=== "Java"
|
||
|
||
```java title="my_heap.java"
|
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[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{siftUp}
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||
```
|
||
|
||
=== "C++"
|
||
|
||
```cpp title="my_heap.cpp"
|
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[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
=== "Python"
|
||
|
||
```python title="my_heap.py"
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||
|
||
```
|
||
|
||
=== "Go"
|
||
|
||
```go title="my_heap.go"
|
||
[class]{maxHeap}-[func]{push}
|
||
|
||
[class]{maxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
=== "JavaScript"
|
||
|
||
```javascript title="my_heap.js"
|
||
[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{#siftUp}
|
||
```
|
||
|
||
=== "TypeScript"
|
||
|
||
```typescript title="my_heap.ts"
|
||
[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
=== "C"
|
||
|
||
```c title="my_heap.c"
|
||
[class]{maxHeap}-[func]{push}
|
||
|
||
[class]{maxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
=== "C#"
|
||
|
||
```csharp title="my_heap.cs"
|
||
[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
=== "Swift"
|
||
|
||
```swift title="my_heap.swift"
|
||
[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
=== "Zig"
|
||
|
||
```zig title="my_heap.zig"
|
||
[class]{MaxHeap}-[func]{push}
|
||
|
||
[class]{MaxHeap}-[func]{siftUp}
|
||
```
|
||
|
||
### 堆顶元素出堆
|
||
|
||
堆顶元素是二叉树根结点,即列表首元素,如果我们直接将首元素从列表中删除,则二叉树中所有结点都会随之发生移位(索引发生变化),这样后续使用堆化修复就很麻烦了。为了尽量减少元素索引变动,采取以下操作步骤:
|
||
|
||
1. 交换堆顶元素与堆底元素(即交换根结点与最右叶结点);
|
||
2. 交换完成后,将堆底从列表中删除(注意,因为已经交换,实际上删除的是原来的堆顶元素);
|
||
3. 从根结点开始,**从顶至底执行堆化**;
|
||
|
||
顾名思义,**从顶至底堆化的操作方向与从底至顶堆化相反**,我们比较根结点的值与其两个子结点的值,将最大的子结点与根结点执行交换,并循环以上操作,直到越过叶结点时结束,或当遇到无需交换的结点时提前结束。
|
||
|
||
=== "<1>"
|
||
![heap_poll_step1](heap.assets/heap_poll_step1.png)
|
||
|
||
=== "<2>"
|
||
![heap_poll_step2](heap.assets/heap_poll_step2.png)
|
||
|
||
=== "<3>"
|
||
![heap_poll_step3](heap.assets/heap_poll_step3.png)
|
||
|
||
=== "<4>"
|
||
![heap_poll_step4](heap.assets/heap_poll_step4.png)
|
||
|
||
=== "<5>"
|
||
![heap_poll_step5](heap.assets/heap_poll_step5.png)
|
||
|
||
=== "<6>"
|
||
![heap_poll_step6](heap.assets/heap_poll_step6.png)
|
||
|
||
=== "<7>"
|
||
![heap_poll_step7](heap.assets/heap_poll_step7.png)
|
||
|
||
=== "<8>"
|
||
![heap_poll_step8](heap.assets/heap_poll_step8.png)
|
||
|
||
=== "<9>"
|
||
![heap_poll_step9](heap.assets/heap_poll_step9.png)
|
||
|
||
=== "<10>"
|
||
![heap_poll_step10](heap.assets/heap_poll_step10.png)
|
||
|
||
与元素入堆操作类似,**堆顶元素出堆操作的时间复杂度为 $O(\log n)$** 。
|
||
|
||
=== "Java"
|
||
|
||
```java title="my_heap.java"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "C++"
|
||
|
||
```cpp title="my_heap.cpp"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "Python"
|
||
|
||
```python title="my_heap.py"
|
||
|
||
```
|
||
|
||
=== "Go"
|
||
|
||
```go title="my_heap.go"
|
||
[class]{maxHeap}-[func]{poll}
|
||
|
||
[class]{maxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "JavaScript"
|
||
|
||
```javascript title="my_heap.js"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{#siftDown}
|
||
```
|
||
|
||
=== "TypeScript"
|
||
|
||
```typescript title="my_heap.ts"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "C"
|
||
|
||
```c title="my_heap.c"
|
||
[class]{maxHeap}-[func]{poll}
|
||
|
||
[class]{maxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "C#"
|
||
|
||
```csharp title="my_heap.cs"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "Swift"
|
||
|
||
```swift title="my_heap.swift"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
=== "Zig"
|
||
|
||
```zig title="my_heap.zig"
|
||
[class]{MaxHeap}-[func]{poll}
|
||
|
||
[class]{MaxHeap}-[func]{siftDown}
|
||
```
|
||
|
||
### 输入数据并建堆 *
|
||
|
||
如果我们想要直接输入一个列表并将其建堆,那么该怎么做呢?最直接地,考虑使用「元素入堆」方法,将列表元素依次入堆。元素入堆的时间复杂度为 $O(\log n)$ ,而平均长度为 $\frac{n}{2}$ ,因此该方法的总体时间复杂度为 $O(n \log n)$ 。
|
||
|
||
然而,存在一种更加优雅的建堆方法。设结点数量为 $n$ ,我们先将列表所有元素原封不动添加进堆,**然后迭代地对各个结点执行「从顶至底堆化」**。当然,**无需对叶结点执行堆化**,因为其没有子结点。
|
||
|
||
=== "Java"
|
||
|
||
```java title="my_heap.java"
|
||
[class]{MaxHeap}-[func]{MaxHeap}
|
||
```
|
||
|
||
=== "C++"
|
||
|
||
```cpp title="my_heap.cpp"
|
||
[class]{MaxHeap}-[func]{MaxHeap}
|
||
```
|
||
|
||
=== "Python"
|
||
|
||
```python title="my_heap.py"
|
||
|
||
```
|
||
|
||
=== "Go"
|
||
|
||
```go title="my_heap.go"
|
||
[class]{maxHeap}-[func]{newMaxHeap}
|
||
```
|
||
|
||
=== "JavaScript"
|
||
|
||
```javascript title="my_heap.js"
|
||
[class]{MaxHeap}-[func]{constructor}
|
||
```
|
||
|
||
=== "TypeScript"
|
||
|
||
```typescript title="my_heap.ts"
|
||
[class]{MaxHeap}-[func]{constructor}
|
||
```
|
||
|
||
=== "C"
|
||
|
||
```c title="my_heap.c"
|
||
[class]{maxHeap}-[func]{newMaxHeap}
|
||
```
|
||
|
||
=== "C#"
|
||
|
||
```csharp title="my_heap.cs"
|
||
[class]{MaxHeap}-[func]{MaxHeap}
|
||
```
|
||
|
||
=== "Swift"
|
||
|
||
```swift title="my_heap.swift"
|
||
[class]{MaxHeap}-[func]{init}
|
||
```
|
||
|
||
=== "Zig"
|
||
|
||
```zig title="my_heap.zig"
|
||
[class]{MaxHeap}-[func]{init}
|
||
```
|
||
|
||
那么,第二种建堆方法的时间复杂度时多少呢?我们来做一下简单推算。
|
||
|
||
- 完全二叉树中,设结点总数为 $n$ ,则叶结点数量为 $(n + 1) / 2$ ,其中 $/$ 为向下整除。因此在排除叶结点后,需要堆化结点数量为 $(n - 1)/2$ ,即为 $O(n)$ ;
|
||
- 从顶至底堆化中,每个结点最多堆化至叶结点,因此最大迭代次数为二叉树高度 $O(\log n)$ ;
|
||
|
||
将上述两者相乘,可得时间复杂度为 $O(n \log n)$ 。然而,该估算结果仍不够准确,因为我们没有考虑到 **二叉树底层结点远多于顶层结点** 的性质。
|
||
|
||
下面我们来尝试展开计算。为了减小计算难度,我们假设树是一个「完美二叉树」,该假设不会影响计算结果的正确性。设二叉树(即堆)结点数量为 $n$ ,树高度为 $h$ 。上文提到,**结点堆化最大迭代次数等于该结点到叶结点的距离,而这正是“结点高度”**。因此,我们将各层的“结点数量 $\times$ 结点高度”求和,即可得到所有结点的堆化的迭代次数总和。
|
||
|
||
$$
|
||
T(h) = 2^0h + 2^1(h-1) + 2^2(h-2) + \cdots + 2^{(h-1)}\times1
|
||
$$
|
||
|
||
![heapify_count](heap.assets/heapify_count.png)
|
||
|
||
化简上式需要借助中学的数列知识,先对 $T(h)$ 乘以 $2$ ,易得
|
||
|
||
$$
|
||
\begin{aligned}
|
||
T(h) & = 2^0h + 2^1(h-1) + 2^2(h-2) + \cdots + 2^{h-1}\times1 \newline
|
||
2 T(h) & = 2^1h + 2^2(h-1) + 2^3(h-2) + \cdots + 2^{h}\times1 \newline
|
||
\end{aligned}
|
||
$$
|
||
|
||
**使用错位相减法**,令下式 $2 T(h)$ 减去上式 $T(h)$ ,可得
|
||
|
||
$$
|
||
2T(h) - T(h) = T(h) = -2^0h + 2^1 + 2^2 + \cdots + 2^{h-1} + 2^h
|
||
$$
|
||
|
||
观察上式,$T(h)$ 是一个等比数列,可直接使用求和公式,得到时间复杂度为
|
||
|
||
$$
|
||
\begin{aligned}
|
||
T(h) & = 2 \frac{1 - 2^h}{1 - 2} - h \newline
|
||
& = 2^{h+1} - h \newline
|
||
& = O(2^h)
|
||
\end{aligned}
|
||
$$
|
||
|
||
进一步地,高度为 $h$ 的完美二叉树的结点数量为 $n = 2^{h+1} - 1$ ,易得复杂度为 $O(2^h) = O(n)$。以上推算表明,**输入列表并建堆的时间复杂度为 $O(n)$ ,非常高效**。
|
||
|
||
## 堆常见应用
|
||
|
||
- **优先队列**。堆常作为实现优先队列的首选数据结构,入队和出队操作时间复杂度为 $O(\log n)$ ,建队操作为 $O(n)$ ,皆非常高效。
|
||
- **堆排序**。给定一组数据,我们使用其建堆,并依次全部弹出,则可以得到有序的序列。当然,堆排序一般无需弹出元素,仅需每轮将堆顶元素交换至数组尾部并减小堆的长度即可。
|
||
- **获取最大的 $k$ 个元素**。这既是一道经典算法题目,也是一种常见应用,例如选取热度前 10 的新闻作为微博热搜,选取前 10 销量的商品等。
|