题目描述

https://www.acwing.com/problem/content/790/

C++ 代码

#include<iostream>
#include<cstdio>
using namespace std;
typedef long long LL;
const int N=100010;
int n,q[N],tmp[N];
LL msort(int l,int r)
{
    if(l>=r) return 0;
    int mid=l+r>>1;
    LL res=msort(l,mid)+msort(mid+1,r);
    int k=0,i=l,j=mid+1;
    while(i<=mid&&j<=r)
        if(q[i]<=q[j]) tmp[k++]=q[i++];
        else {tmp[k++]=q[j++]; res+=mid-i+1;}
    while(i<=mid) tmp[k++]=q[i++];
    while(j<=r) tmp[k++]=q[j++];
    for(int i=l,j=0;i<=r;i++,j++) q[i]=tmp[j];
    return res;
}
int main()
{
    cin>>n;
    for(int i=0;i<n;i++) cin>>q[i];
    cout<<msort(0,n-1);
    return 0;
}