倍增法求LCA

#include<iostream>
#include<algorithm>
#include<cstring>
#include<vector>
#include<set>
#include<map>
#include<queue>
using namespace std;
#define INF 0x3f3f3f3f
#define M(a, b) memset(a, b, sizeof(a))
const int N = 1e4 + 10, M = 20;
int deep[N], p[N][25];
vector<int> G[N];

void dfs(int u, int fa) {    
    for (int i = 1; i <= M; ++i)
        p[u][i] = p[p[u][i-1]][i-1];
    for (int i = 0; i < G[u].size(); ++i) {
        int v = G[u][i];
        if (v == fa || deep[v]) continue;
        deep[v] = deep[u] + 1;
        p[v][0] = u;
        dfs(v, u);
    }
}

int lca(int a, int b) {
    if (deep[a] < deep[b]) swap(a, b);
    for (int det = deep[a]-deep[b], i = 0; det; det >>= 1, ++i)
        if (det & 1) a = p[a][i];
    if (a == b) return a;
    for (int i = M-1; i >= 0; --i) {
        if (p[a][i] == p[b][i]) continue;
        a = p[a][i];
        b = p[b][i];
    }
    return p[a][0];
}