归并排序求解逆序对

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<iostream>
#include<algorithm>
using namespace std;
const int N=500000;
unsigned long int n,a[N],c[N],ans;
void ready()
{
  cin>>n;
  for(int i=1;i<=n;i++)
    cin>>a[i];
}
inline void msort(unsigned long int l,unsigned long int r)
{
  if(l==r)return;
    unsigned long int mid=(l+r)>>1;//">>"表示右移一位，即除2取整
  msort(l,mid);msort(mid+1,r);
    unsigned long int i=l,j=mid+1,k=l;
  while(i<=mid&&j<=r){
    if(a[i]<=a[j])
      c[k++]=a[i++];
    else{
      c[k++]=a[j++];
      ans+=(mid-i+1);//归并中，每当a[i]>a[j]时，就会出现mid-i+1个逆序数。
    }
  }
  while(i<=mid)c[k++]=a[i++];
  while(j<=r)c[k++]=a[j++];
  for(i=l;i<=r;i++)
    a[i]=c[i];
}
void work()
{
  msort(1,n);
  cout<<ans<<endl;
}
int main()
{
  std::ios::sync_with_stdio(false);
  ready();work();return 0;
}

 过oj用。