AcWing 788. 逆序对的数量
原题链接
简单
作者:
术
,
2020-12-30 14:04:27
,
所有人可见
,
阅读 322
#include <iostream>
using namespace std;
const int N=100005;
int n;
int a[N];
int tmp[N];
long long num;
void merge_sort(int a[],int l,int r)
{
if(l>=r) return;
int mid=l+r>>1,i=l,j=mid+1,k=0;
merge_sort(a,l,mid);
merge_sort(a,mid+1,r);
while(i<=mid&&j<=r){
if(a[i]<=a[j]) tmp[k++]=a[i++];
else {
tmp[k++]=a[j++];
num+=mid-i+1;
}
}
while(i<=mid) tmp[k++]=a[i++];
while(j<=r) tmp[k++]=a[j++];
for(int i=l,j=0;i<=r;i++,j++){
a[i]=tmp[j];
}
}
int main()
{
scanf("%d",&n);
for(int i=0;i<n;i++)
scanf("%d",&a[i]);
merge_sort(a,0,n-1);
printf("%lld",num);
//cout << "Hello world!" << endl;
return 0;
}