归并排序求逆序对

代码

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 2e5 + 10;
int a[N],b[N];
ll ans = 0;
#define sc scanf
#define pr printf
#define rep(i,x,y) for(int i = x;i <= y;i++)
#define rvp(i,x,y) for(int i = y;i >= x;i--)
void merge(int l,int r)
{
	if(l == r) return ;
	int m = (l+r) >> 1;
	merge(l,m);
	merge(m+1,r);
	int p1=l,p2 = m + 1;
	int cur = l;
	while(p1 <= m || p2 <= r)
	{
		if(p2 > r|| p1 <= m && a[p1] <= a[p2])
		{
			b[cur++] = a[p1];
			p1++;
		}
		else{
			b[cur++] = a[p2];
			p2++;
			ans += (m - p1 + 1);
		}
	}
	rep(i,l,r) a[i] = b[i];
}

int main()
{
	int n;
	while(~sc("%d",&n))
	{
		rep(i,1,n) sc("%d",&a[i]);
		ans = 0;
		merge(1,n);
		rep(i,1,n) pr("%d ",a[i]);
		puts("");
		pr("%lld\n",ans);
	}
	return 0;
}