LCA Queries on Trees - HackerEarth Trees

#include <bits/stdc++.h>
using namespace std;
#define int long long
vector<int> g[100001];
int sub[100001];
int par[100001];
int dfs(int u,int p = -1)
{
	int count = 1;
	par[u] = par;
	for(auto v : g[u])
	{
		if(v!=p)
		{	
			count += dfs(v);
		}
	}
	return sub[u] = count;
}
signed main()
{
	memset(sub,0,sizeof sub);
	int n;
	cin >> n;
	int z = n-1;
	int x;
	cin >> x;
	while(z--)
	{
		int u,v;
		cin >> u >> v;
		g[u].push_back(v);
		g[v].push_back(u);
	}
	dfs(1);
	int multiplier = 0;
	int ans = 0;
	for(auto v:g[x])
	{
		if(v == par[x])
			continue;
		ans += sub[v];
		ans += multiplier * sub[v];
		multiplier += sub[v];
	}

	cout << ans << '\n';

}