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;
}