AcWing 788. 逆序对的数量
原题链接
简单
#include<iostream>
using namespace std;
typedef long long LL ;
const int N = 1e6 + 10;
int n,q[N],tmp[N];
LL merge_sort(int q[],int l,int r){
if(l>=r)return 0;
int mid = (l+r)>>1;
LL res = 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++];
res += mid - i + 1;
}
}
while(i<=mid) tmp[k++]=q[i++];
while(j<=r) tmp[k++]=q[j++];
for(int i = l,j =0;i<=r;i++,j++) q[i]=tmp[j];
return res;
}
int main(){
scanf("%d", &n);
for(int i =0;i<n;i++) scanf("%d",&q[i]);
LL res = merge_sort(q,0,n-1);
cout<< res <<endl;
}