107. 超快速排序

Posted 2021-09-05 幽殇默

https://www.acwing.com/problem/content/109/

#include<bits/stdc++.h>
using namespace std;
const int N=1e5*5+10;
int a[N],temp[N],n;
long long int res=0;
void merge_sort(int l,int r)
{
    if(l>=r) return;
    int mid=l+r>>1;
    merge_sort(l,mid),merge_sort(mid+1,r);
    int i=l,j=mid+1,k=0;
    while(i<=mid&&j<=r)
    {
        if(a[i]<=a[j]) temp[k++]=a[i++];
        else temp[k++]=a[j++],res+=mid-i+1;
    }
    while(i<=mid) temp[k++]=a[i++];
    while(j<=r) temp[k++]=a[j++];
    for(int i=0,j=l;j<=r;j++,i++) a[j]=temp[i];
}
int main(void)
{
    while(cin>>n,n)
    {
        for(int i=0;i<n;i++) cin>>a[i];
        merge_sort(0,n-1);
        cout<<res<<endl;
        res=0;
    }
    return 0;
}