AtCoder Beginner Contest 014 D: 閉路

解法

AtCoder Beginner Contest 014 解説

LCA のライブラリを蟻本から写経。

コード

#include <bits/stdc++.h>

using namespace std;
typedef signed long long ll;

class LCA {
 private:
  LCA() {}
  vector<vector<int>> G;
  vector<vector<int>> parent;
  vector<int> depth;
  int root, logV;

 public:
  void dfs(int v, int p, int d) {
    parent[0][v] = p;
    depth[v] = d;
    for (auto &u : G[v])
      if (u != p) dfs(u, v, d + 1);
  }

  LCA(const vector<vector<int>> &adj) {
    int V = adj.size();
    root = 0;
    G = adj;
    depth.assign(V, 0);

    logV = 1;
    for (int i = 1; i <= V;) i *= 2, logV++;
    parent.assign(logV, vector<int>(V));

    dfs(root, -1, 0);

    for (int k = 0; k + 1 < logV; ++k)
      for (int v = 0; v < V; ++v)
        if (parent[k][v] < 0) {
          parent[k + 1][v] = -1;
        } else {
          parent[k + 1][v] = parent[k][parent[k][v]];
        }
  }

  int getLCA(int u, int v) {
    if (depth[u] > depth[v]) swap(u, v);
    for (int k = 0; k < logV; ++k)
      if ((depth[v] - depth[u]) >> k & 1) v = parent[k][v];
    if (u == v) return u;
    for (int k = logV - 1; k >= 0; --k)
      if (parent[k][u] != parent[k][v]) {
        u = parent[k][u];
        v = parent[k][v];
      }
    return parent[0][u];
  }

  int getLength(int u, int v) {
    int lca = getLCA(u, v);
    return depth[u] + depth[v] - depth[lca] * 2;
  }
};

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N;
  cin >> N;
  vector<vector<int>> adj(N, vector<int>());
  for (int i = 0; i < N - 1; ++i) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    adj[x].push_back(y);
    adj[y].push_back(x);
  }

  LCA lca(adj);
  int Q;
  cin >> Q;
  for (int i = 0; i < Q; ++i) {
    int x, y;
    cin >> x >> y;
    x--, y--;
    cout << lca.getLength(x, y) + 1 << "\n";
  }
}