分治算法——正规归并排序中顺便计算出数组中的逆序对数

C++版本代码如下

#include <iostream>
#include <math.h>
#include <cstring>
using namespace std;

#define MAXSIZE 256

int num = 0; // 全局变量或者成员变量

void merge(int a[], int low, int middle, int high) {
    int i = low, j = middle + 1, k = 0;
    int tmp[high - low + 1];
    while (i<=middle && j<=high) {
        if (a[i] <= a[j])
            tmp[k++] = a[i++];
        else{
            // 统计i与middle之间，比a[j]大的元素个数
            num += (middle - i + 1);
            tmp[k++] = a[j++];
        }
    }

    // 处理剩下的
    while (i <= middle)
        tmp[k++] = a[i++];

    while (j <= high)
        tmp[k++] = a[j++];

    // 从tmp拷贝回a
    for (i = 0; i < high - low + 1; ++i) {
        a[low + i] = tmp[i];
    }
}

void mergeSortCounting(int a[], int low, int high) {
    if (low >= high) return;
    int middle = low + ((high - low)>>1);
    mergeSortCounting(a, low, middle);
    mergeSortCounting(a, middle+1, high);
    merge(a, low, middle, high);
}

int main(int argc, char* argv[]){
    int arr[6] = {1,5,6,2,3,4};
    mergeSortCounting(arr, 0, 5);
    for(int i = 0; i < 6; i++){
        cout<<arr[i]<<"	";
    }
    cout<<endl<<num;
    return 0;
}