归并排序之逆序对

题目传送：#1141 : 二分·归并排序之逆序对

好吧，第一次手敲归并，略挫，不过爆intWA了一发

AC代码：

#include <map>
#include <set>
#include <list>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <cctype>
#include <cstdio>
#include <string>
#include <vector>
#include <complex>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <sstream>
#include <utility>
#include <iostream>
#include <algorithm>
#include <functional>
#define LL long long
#define INF 0x7fffffff
using namespace std;

int a[100005];
int n;
LL ans;//好吧，会爆int

int tmp[100005];

void merge_sort(int s, int t) {
    if(s == t) return;
    int mid = (s + t) >> 1;
    merge_sort(s, mid);
    merge_sort(mid + 1, t);

    int cnt = 0;
    int i, j;
    for(i = s, j = mid + 1; i <= mid && j <= t; ) {
        if(a[i] > a[j]) {
            tmp[cnt ++] = a[j];
            j ++;
            ans += (mid - i + 1);
        }
        else {
            tmp[cnt ++] = a[i];
            i ++;
        }
    }
    while(i <= mid) {
        tmp[cnt ++] = a[i ++];
    }
    while(j <= t) {
        tmp[cnt ++] = a[j ++];
    }
    for(int i = s, j = 0; i <= t && j <= cnt; i ++, j ++) {
        a[i] = tmp[j];
    }
}

int main() {
    scanf("%d", &n);
    for(int i = 1; i <= n; i ++) {
        scanf("%d", &a[i]);
    }
    ans = 0;
    merge_sort(1, n);
    printf("%lld\n", ans);//注意这里用%I64d会PE，或者用cout也行
    //cout << ans << endl;
    return 0;
}