输出逆序数

#include <bits/stdc++.h>#include<algorithm>using namespace std;int a[100010];int b[100010];void Merge(int a[],int s,int m,int e,int tmp[]){int pb=0;int p1=s,p2=m+1;while(p1<=m&&p2<=e){if(a[p1]>a[p2])  tmp[pb++]=a[p1++];else   tmp[pb++]=a[p2++];   }while(p1<=m)   tmp[pb++]=a[p1++];while(p2<=e)   tmp[pb++]=a[p2++];   for(int i=0;i<e-s+1;i++)   a[s+i]=tmp[i];}long long Count(int a[],int s,int m,int e){long long result=0;int p1=s,p2=m+1;while(p1<=m&&p2<=e){if(a[p1]>a[p2]){result+=e-p2+1;++p1;}else      ++p2;}return result;}long long MergeSort(int a[],int s,int e,int tmp[]){        long long result=0;if(s<e){int m=s+(e-s)/2;result+=MergeSort(a,s,m,tmp);result+=MergeSort(a,m+1,e,tmp);result+=Count(a,s,m,e);Merge(a,s,m,e,tmp);//从大到小合并，确保排序 }return result;}int main(){int n;cin>>n;for(int i=0;i<n;++i)      cin>>a[i];  printf("%lld",MergeSort(a,0,n-1,b));  return 0;}