打卡第三天-merge_sort
作者:
夜深忽梦少年事
,
2024-07-22 13:59:54
,
所有人可见
,
阅读 4
重铸华农荣光 我辈义不容辞
打卡第三天-merge_sort
#include<iostream>
using namespace std;
const int N=1000010;
int q[N],tmp[N];
long long ans=0;
void merge_sort(int q[],int l,int r)
{
if(l>=r) return ;
int mid =l+r>>1;
merge_sort(q,l,mid);
merge_sort(q,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++],ans+=mid-i+1;
}
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];
}
int main()
{
int n;
cin>>n;
for(int i=0;i<n;i++) cin>>q[i];
merge_sort(q,0,n-1);
cout<<ans;
return 0;
}