POJ 2299 Ultra-QuickSort

　　题目的意思很明确，就是求逆序对，于是直接在归并排序里加上ans+=mid-i+1求解即可

　　代码如下：

#include <iostream>
#define MAXN 500003

using namespace std;

int N;
long long ans;
int a[MAXN] , t[MAXN];

void    merge(int l , int r) {
    int mid = (l+r) >> 1 , i = l , j = mid+1 , s = l;
    while (s <= r) {
        if (j > r || (i <= mid && a[i] < a[j]))
                t[s++] = a[i++];
            else {
                ans += mid-i+1;
                t[s++] = a[j++];
            }
    }
    for (i = l;i <= r;i++) a[i] = t[i];
}

void    msort(int l , int r) {
    int mid = (l+r) >> 1;
    if (l == r) return ;
    msort(l,mid);
    msort(mid+1,r);
    merge(l,r);
}

int main () {
    while (cin >> N && N) {
        ans = 0;
        for (int i = 0;i < N;i++)
            cin >> a[i];
        msort(0,N-1);
        cout << ans << endl;
    }
    return 0;
}