AcWing 788. 逆序对的数量
原题链接
简单
作者:
月亮事务所
,
2021-02-07 23:56:02
,
所有人可见
,
阅读 286
#include <iostream>
using namespace std;
typedef long long LL;
const int N= 100010;
int n;
int q[N], tmp[N];
LL merge_sort(int l, int r)
{
if(l > r) return 0;
int mid=1+r>> 1;
LL res=merge_sort(l, mid)+ merge_sort(mid+ 1,r);
int k=0,i=1,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()
{
cin >> n;
for (int i=0;i<n;i ++) cin >> q[i];
cout << merge_sort(0, n- 1)<< endl;
return 0;
}