python/TangDou/AcWing/LCA/P3379_BeiZeng.cpp

#include <bits/stdc++.h>
using namespace std;
const int N = 500010, M = N << 1;

int e[M], h[N], idx, w[M], ne[M];
void add(int a, int b, int c = 0) {
    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
}

int depth[N];
int f[N][20];

void bfs(int root) {
    queue<int> q;
    q.push(root);
    depth[root] = 1;
    while (q.size()) {
        int u = q.front();
        q.pop();
        for (int i = h[u]; ~i; i = ne[i]) {
            int v = e[i];
            if (!depth[v]) {
                depth[v] = depth[u] + 1;
                f[v][0] = u;
                q.push(v);

                for (int k = 1; k <= 19; k++) f[v][k] = f[f[v][k - 1]][k - 1];
            }
        }
    }
}

int lca(int a, int b) {
    if (depth[a] < depth[b]) swap(a, b);

    for (int k = 19; k >= 0; k--)
        if (depth[f[a][k]] >= depth[b]) a = f[a][k];

    if (a == b) return a;

    for (int k = 19; k >= 0; k--)
        if (f[a][k] != f[b][k])
            a = f[a][k], b = f[b][k];

    return f[a][0];
}

int main() {
#ifndef ONLINE_JUDGE
    freopen("P3379.in", "r", stdin);
#endif
    memset(h, -1, sizeof h);

    // Q:为什么上面的数组开20，每次k循环到19？
    // 答： cout << log2(500000) << endl;

    int n, m, s;
    scanf("%d %d %d", &n, &m, &s);

    int a, b;
    for (int i = 1; i < n; i++) {
        scanf("%d %d", &a, &b);
        add(a, b), add(b, a);
    }

    bfs(s);

    while (m--) {
        scanf("%d %d", &a, &b);
        cout << lca(a, b) << endl;
    }
    return 0;
}