【机房——LCA】

题目

代码

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

const int N = 1e5+10;
const int M = 2e5+10;

int n, q, cnt[N];
int h[N], e[M], ne[M], idx;
int d[N], f[N][18], dist[N][18];

void add(int a, int b)
{
	e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}
void dfs(int u, int fa)
{
	for(int i = h[u]; ~i; i = ne[i])
	{
		int j = e[i];
		if(j == fa) continue;
		d[j] = d[u] + 1;
		f[j][0] = u;
		dist[j][0] = cnt[u];
		for(int k = 1; k <= 17; k++)
		{
			f[j][k] = f[f[j][k-1]][k-1];
			dist[j][k] = dist[j][k-1] + dist[f[j][k-1]][k-1];
		}
		
		dfs(j, u);
	}
}
int lca(int a, int b)
{
	int retv = cnt[a] + cnt[b];
	
	if(d[a] < d[b]) swap(a, b);
	
	for(int i = 17; i >= 0; i--)
		if(d[f[a][i]] >= d[b])
		{
			retv += dist[a][i];
			a = f[a][i];
		}

	if(a == b) return retv - cnt[a];
	
	for(int i = 17; i >= 0; i--)
		if(f[a][i] != f[b][i])
		{
			retv += dist[a][i];
			retv += dist[b][i];
			a = f[a][i];
			b = f[b][i];
		}
		
	retv += dist[a][0] + dist[b][0];
	return retv - cnt[f[a][0]];
}
int main()
{
	memset(h, -1, sizeof h);
	scanf("%d%d", &n, &q);
	
	for(int i = 1; i < n; i++)
	{
		int a, b;
		scanf("%d%d", &a, &b);
		add(a, b); add(b, a);
		cnt[a]++, cnt[b]++;
	}
	
	d[1] = 1;
	dfs(1, -1);
	
	while(q--)
	{
		int a, b;
		scanf("%d%d", &a, &b);
		printf("%d\n", lca(a, b));
	}
}