以前用树状数组求逆序对,现在用归并排序写一下,顺便复习一下归并排序.
#include<iostream>
#include<cmath>
#include<algorithm>
#include<cstring>
#include<vector>
#include<string>
#define maxn 0x3f3f3f3f
#define ll long long
using namespace std;
int a[500005];
int b[500005];
ll cnt;
void merge(int l,int mid,int r){
int i=l,j=mid+1;
for(int k=l;k<=r;++k){
if(j>r||i<=mid&&a[i]<=a[j]) b[k]=a[i++];
else b[k]=a[j++],cnt+=mid-i+1;
}
for(int k=l;k<=r;++k) a[k]=b[k];
}
void mergeSort(int l, int r){
if(l<r){
int mid=(l+r)>>1;
mergeSort(l,mid);
mergeSort(mid+1,r);
merge(l,mid,r);
}
}
int main(){
int n;
cin>>n;
for(int i=1;i<=n;++i){
scanf("%d",&a[i]);
}
mergeSort(1,n);
cout<<cnt<<endl;
return 0;
}