poj1330 Nearest Common Ancestors P(LCA)

#poj1330 Nearest Common Ancestors P(LCA)
###传送门：http://poj.org/problem?id=1330

#include<iostream>
#include<cstdio>
#include<cstring>
#include<vector>
using namespace std;
const int N = 1e4 + 10;

int _,n;
int dfn[N],du[N],pre[N][30],lg[N];
vector<int>q[N];

void init()
{
	for(int i = 0; i <= n + 5; i++){
		q[i].clear();
		dfn[i] = 0;
		du[i] = 0;
	}
	memset(pre,0,sizeof pre);
}

void dfs(int u,int fa)
{
	dfn[u] = dfn[fa] + 1;
	pre[u][0] = fa;
	for(int i = 1; (1 << i) <= dfn[u]; ++i)
		pre[u][i] = pre[pre[u][i - 1]][i - 1];
	for(int j = 0; j < q[u].size(); j++){
		int v = q[u][j];
		if(v != fa)dfs(v,u);
	}
}

int lca(int x,int y)
{
	if(dfn[x] < dfn[y])swap(x,y);
	while(dfn[x] > dfn[y])
		x = pre[x][lg[dfn[x] - dfn[y]] - 1];
	if(x == y)return x;
	for(int k = lg[dfn[x]]; k >= 0; --k)
		if(pre[x][k] != pre[y][k])
			x = pre[x][k],y = pre[y][k];
	return pre[x][0];
}

int main()
{
	int _;
	cin >> _;
	for(int i = 1; i < N; i++)lg[i] = lg[i - 1] + (1 << lg[i - 1] == i);
	while(_--){
		init();
		cin >> n;
		for(int i = 1; i < n; i++){
			int a,b;
			cin >> a >> b;
			q[a].push_back(b);
			du[b]++;
		}
		for(int i = 1; i <= n; ++i){
			if(du[i] == 0){
				dfs(i,0);
				break;
			}
		}
		int x,y;
		cin >> x >> y;
		cout << lca(x,y) << endl;
	}
	return 0;
}