build

chapter_computational_complexity/time_complexity.md

@@ -1516,7 +1516,7 @@ $$
     func bubbleSort(nums: inout [Int]) -> Int {
         var count = 0 // 计数器
         // 外循环：待排序元素数量为 n-1, n-2, ..., 1
-        for i in sequence(first: nums.count - 1, next: { $0 > 0 + 1 ? $0 - 1 : nil }) {
+        for i in stride(from: nums.count - 1, to: 0, by: -1) {
             // 内循环：冒泡操作
             for j in 0 ..< i {
                 if nums[j] > nums[j + 1] {
@@ -2221,7 +2221,7 @@ $$
             return 1
         }
         var count = linearLogRecur(n: n / 2) + linearLogRecur(n: n / 2)
-        for _ in sequence(first: 0, next: { $0 < n - 1 ? $0 + 1 : nil }) {
+        for _ in stride(from: 0, to: n, by: 1) {
             count += 1
         }
         return count

chapter_sorting/counting_sort.md

@@ -158,7 +158,26 @@ comments: true
 === "Swift"
 
     ```swift title="counting_sort.swift"
-    [class]{}-[func]{countingSortNaive}
+    /* 计数排序 */
+    // 简单实现，无法用于排序对象
+    func countingSortNaive(nums: inout [Int]) {
+        // 1. 统计数组最大元素 m
+        let m = nums.max()!
+        // 2. 统计各数字的出现次数
+        // counter[num] 代表 num 的出现次数
+        var counter = Array(repeating: 0, count: m + 1)
+        for num in nums {
+            counter[num] += 1
+        }
+        // 3. 遍历 counter ，将各元素填入原数组 nums
+        var i = 0
+        for num in stride(from: 0, to: m + 1, by: 1) {
+            for _ in stride(from: 0, to: counter[num], by: 1) {
+                nums[i] = num
+                i += 1
+            }
+        }
+    }
     ```
 
 === "Zig"
@@ -381,7 +400,35 @@ $$
 === "Swift"
 
     ```swift title="counting_sort.swift"
-    [class]{}-[func]{countingSort}
+    /* 计数排序 */
+    // 完整实现，可排序对象，并且是稳定排序
+    func countingSort(nums: inout [Int]) {
+        // 1. 统计数组最大元素 m
+        let m = nums.max()!
+        // 2. 统计各数字的出现次数
+        // counter[num] 代表 num 的出现次数
+        var counter = Array(repeating: 0, count: m + 1)
+        for num in nums {
+            counter[num] += 1
+        }
+        // 3. 求 counter 的前缀和，将“出现次数”转换为“尾索引”
+        // 即 counter[num]-1 是 num 在 res 中最后一次出现的索引
+        for i in stride(from: 0, to: m, by: 1) {
+            counter[i + 1] += counter[i]
+        }
+        // 4. 倒序遍历 nums ，将各元素填入结果数组 res
+        // 初始化数组 res 用于记录结果
+        var res = Array(repeating: 0, count: nums.count)
+        for i in stride(from: nums.count - 1, through: 0, by: -1) {
+            let num = nums[i]
+            res[counter[num] - 1] = num // 将 num 放置到对应索引处
+            counter[num] -= 1 // 令前缀和自减 1 ，得到下次放置 num 的索引
+        }
+        // 使用结果数组 res 覆盖原数组 nums
+        for i in stride(from: 0, to: nums.count, by: 1) {
+            nums[i] = res[i]
+        }
+    }
     ```
 
 === "Zig"