逆序数（归并排序 ）

 题目链接：
 http://acm.nyist.net/JudgeOnline/problem.php?pid=117 
 1 #include <iostream>
#include <cstdio>
using namespace std;

int a[1000001], b[1000001];
long long count;　

void Merge(int i, int m, int j){
    int k, x, y;
    y = m;
    for(k = i; k <= j; ++k) b[k] = a[k];
    for(k = i, x = m + 1; i <= m && x <= j; k++){
        if(b[i] <= b[x]) a[k] = b[i++];
        else{
            a[k] = b[x++];
            count = count + m - i + 1;
        }
    }
    while(i <= m) a[k++] = b[i++];
    while(x <= j) a[k++] = b[x++];
}

void Msort(int i, int j){
    if(i < j){
        int m = (i + j) / 2;
        Msort(i, m);
        Msort(m + 1, j);
        Merge(i, m, j);
    }
}

int main()
{
    int T, i, j, k, N;
    scanf("%d",&T);
    while(T--){
        scanf("%d", &N);
        for(i = 1; i <= N; ++i) scanf("%d",&a[i]);
        count = 0;
        Msort(1, N);
        printf("%lld
", count);
    }
    return 0;
}