hello-algo/codes/cpp/chapter_computational_complexity/time_complexity.cpp

/**
 * File: time_complexity.cpp
 * Created Time: 2022-11-25
 * Author: Krahets (krahets@163.com)
 */

#include "../utils/common.hpp"

/* 常数阶 */
int constant(int n) {
    int count = 0;
    int size = 100000;
    for (int i = 0; i < size; i++)
        count++;
    return count;
}

/* 线性阶 */
int linear(int n) {
    int count = 0;
    for (int i = 0; i < n; i++)
        count++;
    return count;
}

/* 线性阶（遍历数组） */
int arrayTraversal(vector<int> &nums) {
    int count = 0;
    // 循环次数与数组长度成正比
    for (int num : nums) {
        count++;
    }
    return count;
}

/* 平方阶 */
int quadratic(int n) {
    int count = 0;
    // 循环次数与数组长度成平方关系
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < n; j++) {
            count++;
        }
    }
    return count;
}

/* 平方阶（冒泡排序） */
int bubbleSort(vector<int> &nums) {
    int count = 0; // 计数器
    // 外循环：未排序区间为 [0, i]
    for (int i = nums.size() - 1; i > 0; i--) {
        // 内循环：将未排序区间 [0, i] 中的最大元素交换至该区间的最右端 
        for (int j = 0; j < i; j++) {
            if (nums[j] > nums[j + 1]) {
                // 交换 nums[j] 与 nums[j + 1]
                int tmp = nums[j];
                nums[j] = nums[j + 1];
                nums[j + 1] = tmp;
                count += 3; // 元素交换包含 3 个单元操作
            }
        }
    }
    return count;
}

/* 指数阶（循环实现） */
int exponential(int n) {
    int count = 0, base = 1;
    // 细胞每轮一分为二，形成数列 1, 2, 4, 8, ..., 2^(n-1)
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < base; j++) {
            count++;
        }
        base *= 2;
    }
    // count = 1 + 2 + 4 + 8 + .. + 2^(n-1) = 2^n - 1
    return count;
}

/* 指数阶（递归实现） */
int expRecur(int n) {
    if (n == 1)
        return 1;
    return expRecur(n - 1) + expRecur(n - 1) + 1;
}

/* 对数阶（循环实现） */
int logarithmic(float n) {
    int count = 0;
    while (n > 1) {
        n = n / 2;
        count++;
    }
    return count;
}

/* 对数阶（递归实现） */
int logRecur(float n) {
    if (n <= 1)
        return 0;
    return logRecur(n / 2) + 1;
}

/* 线性对数阶 */
int linearLogRecur(float n) {
    if (n <= 1)
        return 1;
    int count = linearLogRecur(n / 2) + linearLogRecur(n / 2);
    for (int i = 0; i < n; i++) {
        count++;
    }
    return count;
}

/* 阶乘阶（递归实现） */
int factorialRecur(int n) {
    if (n == 0)
        return 1;
    int count = 0;
    // 从 1 个分裂出 n 个
    for (int i = 0; i < n; i++) {
        count += factorialRecur(n - 1);
    }
    return count;
}

/* Driver Code */
int main() {
    // 可以修改 n 运行，体会一下各种复杂度的操作数量变化趋势
    int n = 8;
    cout << "输入数据大小 n = " << n << endl;

    int count = constant(n);
    cout << "常数阶的操作数量 = " << count << endl;

    count = linear(n);
    cout << "线性阶的操作数量 = " << count << endl;
    vector<int> arr(n);
    count = arrayTraversal(arr);
    cout << "线性阶（遍历数组）的操作数量 = " << count << endl;

    count = quadratic(n);
    cout << "平方阶的操作数量 = " << count << endl;
    vector<int> nums(n);
    for (int i = 0; i < n; i++)
        nums[i] = n - i; // [n,n-1,...,2,1]
    count = bubbleSort(nums);
    cout << "平方阶（冒泡排序）的操作数量 = " << count << endl;

    count = exponential(n);
    cout << "指数阶（循环实现）的操作数量 = " << count << endl;
    count = expRecur(n);
    cout << "指数阶（递归实现）的操作数量 = " << count << endl;

    count = logarithmic((float)n);
    cout << "对数阶（循环实现）的操作数量 = " << count << endl;
    count = logRecur((float)n);
    cout << "对数阶（递归实现）的操作数量 = " << count << endl;

    count = linearLogRecur((float)n);
    cout << "线性对数阶（递归实现）的操作数量 = " << count << endl;

    count = factorialRecur(n);
    cout << "阶乘阶（递归实现）的操作数量 = " << count << endl;

    return 0;
}