ACM模板~求逆序对的个数

#include <map>
#include <set>
#include <cmath>
#include <ctime>
#include <stack>
#include <queue>
#include <cstdio>
#include <cctype>
#include <bitset>
#include <string>
#include <vector>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <functional>
#define fuck(x) cout<<"["<<x<<"]";
#define FIN freopen("input.txt","r",stdin);
#define FOUT freopen("output.txt","w+",stdout);
#pragma comment(linker, "/STACK:102400000,102400000")
using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
const int maxn = 1e5+5;
int a[maxn];  //原始数组
int c[maxn]; //维护
long long cnt=0;  //答案
void Msort(int l,int r){
    int mid=(l+r)/2;
    int i,j,tmp;
    if(r>l){
        Msort(l,mid);
        Msort(mid+1,r);
        tmp=l;
        for(i=l,j=mid+1;i<=mid&&j<=r; ){
            if(a[i]>a[j]){
                c[tmp++]=a[j++];
                cnt+=mid-i+1;
            }else{
                c[tmp++]=a[i++];
            }
        }
        if(i<=mid) for(;i<=mid;) c[tmp++]=a[i++];
        if(j<=r) for(;j<=r;) c[tmp++]=a[j++];
        for(i=l;i<=r;i++) a[i]=c[i];
    }
}

int main(){
    int n,t;
    while(scanf("%d",&n) !=EOF){
        cnt=0;
        for(int i=0;i<n;i++){
            scanf("%d",&a[i]);
        }
        Msort(0,n-1);
        cout<<cnt<<endl;
    }
    return 0;
}