求数组逆序对

思路：类似归并排序算法，在合并已经有序的相邻子数组的时候，计算前面数组相对于后面数组的逆序对数，整个递归过程可以算出所有逆序对
#include <stdio.h>
void merge(int A[], int front, int middle, int end, int &count)
{
    if (front >= end) return;
    int i = front;
    int j = middle + 1;
    int k = 0;
    int *p = new int[end - front + 1];
    while (i <= middle && j <= end) {
        if (A[i] < A[j]) {
            p[k++] = A[i++];
        } else {
            count += middle - i + 1;
            p[k++] = A[j++];
        }
    }
    if (j <= end) while (j <= end) {
        p[k++] = A[j++];
        count += middle - i + 1;
    }
    if (i <= middle) while (i <= middle) p[k++] = A[i++];
    printf("front:%d end:%d :", front, end);
    for (int i = 0; i < k; ++i) {
        printf("%d ", p[i]);
        A[front + i] = p[i];
    }
    printf("
");

    delete [] p;
}
void merge_sort(int A[], int front, int end, int &count)
{
    if (front >= end) return;
    int middle = (front + end) / 2;
    merge_sort(A, front, middle, count);
    merge_sort(A, middle + 1, end, count);
    merge(A, front, middle, end, count);
}

int main()
{
    int count = 0;
    int rand[] = {3,1,5,23,2,32,56,76,65,34,2,2,333,1,0};
    merge_sort(rand, 0, 4,  count);
    for (int i = 0; i < 5; ++i) {
        printf("%d ", rand[i]);
    }
    printf("
");
    printf("%d
", count);
    return 0;
}