#include<bits/stdc++.h>
using namespace std;
const int N=1e+5;
int a[N],tmp[N];
typedef long long ll; #注意题目条件
ll merge_sort(int q[],int l,int r){
if(l>=r) return 0;
int mid=l+r>>1;
ll res=merge_sort(q,l,mid)+merge_sort(q,mid+1,r); #注意
int k=0,i=l,j=mid+1;
while(i<=mid&&j<=r){
if(q[i]<=q[j])
tmp[k++]=q[i++];
else{
res+=mid-i+1; #回忆计算过程(力扣视频上有)
tmp[k++]=q[j++];
}
}
while(i<=mid)
tmp[k++]=q[i++];
while(j<=r)
tmp[k++]=q[j++];
for(i=l,j=0;i<=r;i++,j++)
q[i]=tmp[j];
return res;
}
int main(){
int n;
cin>>n;
for (int i = 0; i < n; i ++ ) scanf("%d", &a[i]);
cout<<merge_sort(a,0,n-1)<<endl;
return 0;
}
标签:sort,int,ll,788,mid,merge,逆序,Acwing
From: https://www.cnblogs.com/Nikkie-02/p/16830813.html