归并排序与逆序对

归并

逆序对

#include<iostream>
#include<cstdio>
#include<cctype>
using namespace std;
inline long long read()
{
    long long x=0,f=1;char c=getchar();
    while(!isdigit(c)){if(c=='-')f=-1;c=getchar();}
    while(isdigit(c)){x=(x<<3)+(x<<1)+(c^48);c=getchar();}
    return x*f;
}
long long a[500001],r[500001],n,ans;
inline void msort(int i,int j)
{
    if(i==j)return;
    int mid=i+j>>1;
    msort(i,mid);
    msort(mid+1,j);
    int x=i,y=mid+1;
    for(int k=i;k<=j;k++)
    if((y>j)||(x<=mid&&a[x]<=a[y]))r[k]=a[x++];
    else r[k]=a[y++],ans+=mid-x+1;
    for(int p=i;p<=j;p++)a[p]=r[p];
    return;
}
int main()
{
    n=read();
    for(int i=1;i<=n;i++)
    a[i]=read();
    msort(1,n);
    printf("%lld\n",ans);
    return 0;
}