用归并排序求逆序对个数

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
const int N = 100000+5;
int a[N],tmp[N];
ll ans;

void Merge(int l,int r)
{
	int m = (l + r) >> 1;
    int i = l;
    int j = m + 1;
    int k = l;  //特别注意，k是从l（字母）开始而不是0；
    while(i <= m && j <= r)
    {
        if(a[i] > a[j])
        {
            tmp[k++] = a[j++];
            ans += m - i + 1;
        }
        else
        {
            tmp[k++] = a[i++];
        }
    }
    while(i <= m) tmp[k++] = a[i++];
    while(j <= r) tmp[k++] = a[j++];
    for(int i=l;i<=r;i++)
        a[i] = tmp[i];
}

void Merge_sort(int l,int r)
{
    if(l < r)
    {
        int m = (l + r) >> 1;
        Merge_sort(l,m);
        Merge_sort(m+1,r);
        Merge(l,r);
    }
}

int main()
{
    int n;
    while(scanf("%d",&n)==1)
    {
	    for(int i=0;i<n;i++)
	        scanf("%d",&a[i]);
	    ans = 0;
	    Merge_sort(0,n-1);
	    printf("%lld\n",ans);
	}
    return 0;
}