107. 超快速排序归并 / 逆序对的数量

Posted 2022-03-16 辉小歌

#include<bits/stdc++.h>
using namespace std;
const int N=1e5*5+10;
int a[N],b[N],n;
long long int ans;
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]) b[k++]=a[i++];
        else 
        
            ans+=mid-i+1;
            b[k++]=a[j++];
        
    
    while(i<=mid) b[k++]=a[i++];
    while(j<=r) b[k++]=a[j++];
    for(int i=l,j=0;i<=r;i++,j++) a[i]=b[j];

int main(void)

    while(cin>>n,n)
    
        for(int i=0;i<n;i++) scanf("%d",&a[i]);
        ans=0;
        merge_sort(0,n-1);
        printf("%lld\\n",ans);
    
    return 0;