AcWing 788. 逆序对的数量
原题链接
简单
作者:
a_little
,
2024-04-10 17:37:53
,
所有人可见
,
阅读 1
#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
typedef long long LL;
const int N = 100010;
int a[N],t[N];
LL res;
LL merge_sort(int l,int r){
if(l>=r) return 0;
int mid=l+r>>1;
merge_sort(l,mid),merge_sort(mid+1,r);
int k=0,i=l,j=mid+1;
while(i<=mid&&j<=r){
if(a[i]<=a[j]) t[k++]=a[i++];
else{
t[k++]=a[j++];
res+=mid-i+1;
}
}
while(i<=mid) t[k++]=a[i++];
while(j<=r) t[k++]=a[j++];
for(int i=l,k=0;i<=r;i++,k++) a[i]=t[k];
return res;
}
int main(){
int n;
cin>>n;
for(int i=0;i<n;i++) scanf("%d",&a[i]);
cout<<merge_sort(0,n-1);
return 0;
}