1277E - Two Fairs - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;

const int maxn=5e5+10;
int cnt,cnta,cntb;
int vis[maxn];
vector <int> e[maxn];

void dfs(int k,int t)
{
	vis[k]=t;cnt++;
	for (auto i:e[k]) if (vis[i]!=t) dfs(i,t);
}

int main()
{
	int T;scanf("%d",&T);
	for (int t=1;t<=T;t++)
	{
		int n,m,a,b;scanf("%d%d%d%d",&n,&m,&a,&b);
		for (int i=1;i<=n;i++) e[i].clear();
		for (int i=1;i<=m;i++)
		{
			int x,y;scanf("%d%d",&x,&y);
			e[x].push_back(y);
			e[y].push_back(x);
		}
		
		vis[a]=t*2-1;cnt=1;dfs(b,t*2-1);cnta=n-cnt;
		vis[b]=t*2;cnt=1;dfs(a,t*2);cntb=n-cnt;
		printf("%lld\n",1ll*cnta*cntb);
	}
	
	return 0;
}