Problem p02373 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <iostream>
#include <vector>

#define REP(i, n) for (int i = 0; i < int(n); ++i)
#define FOR(i, a, b) for (int i = int(a); i < int(b); ++i)
#define EACH(i, c)                                                             \
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define ALL(A) A.begin(), A.end()

using namespace std;

struct HeavyLight {
  vector<vector<int>> tree;
  int pathCount, n;
  vector<int> size, parent, in, out, path, pathSize, pathPos, pathRoot;

  HeavyLight(vector<vector<int>> t)
      : tree(t), n(tree.size()), size(n), parent(n), in(n), out(n), path(n),
        pathSize(n), pathPos(n), pathRoot(n) {
    int time = 0;
    dfs(0, -1, time);
    buildPaths(0, newPath(0));
  }

  void dfs(int u, int p, int &k) {
    in[u] = k++, parent[u] = p, size[u] = 1;
    EACH(v, tree[u]) if (*v != p) dfs(*v, u, k), size[u] += size[*v];
    out[u] = k++;
  }

  int newPath(int u) {
    pathRoot[pathCount] = u;
    return pathCount++;
  }

  void buildPaths(int u, int pt) {
    path[u] = pt, pathPos[u] = pathSize[pt]++;
    EACH(v, tree[u])
    if (*v != parent[u])
        buildPaths(*v, 2 * size[*v] >= size[u] ? pt : newPath(*v));
  }

  bool isAncestor(int p, int ch) {
    return in[p] <= in[ch] && out[ch] <= out[p];
  }

  int lca(int a, int b) {
    for (int root; !isAncestor(root = pathRoot[path[a]], b); a = parent[root])
      ;
    for (int root; !isAncestor(root = pathRoot[path[b]], a); b = parent[root])
      ;
    return isAncestor(a, b) ? a : b;
  }
};

// Usage: http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=GRL_5_C
int main(void) {
  // ios::sync_with_stdio(false);

  int n;
  cin >> n;
  vector<vector<int>> tree(n);

  REP(i, n) {
    int k;
    cin >> k;
    REP(j, k) {
      int ch;
      cin >> ch;
      tree[i].push_back(ch);
    }
  }

  HeavyLight hl = HeavyLight(tree);

  int q;
  cin >> q;
  while (q--) {
    int u, v;
    cin >> u >> v;
    cout << hl.lca(u, v) << "\n";
  }

  return 0;
}

Fixed submission:
#include <algorithm>
#include <iostream>
#include <vector>

#define REP(i, n) for (int i = 0; i < int(n); ++i)
#define FOR(i, a, b) for (int i = int(a); i < int(b); ++i)
#define EACH(i, c)                                                             \
  for (__typeof((c).begin()) i = (c).begin(); i != (c).end(); ++i)
#define ALL(A) A.begin(), A.end()

using namespace std;

struct HeavyLight {
  vector<vector<int>> len, tree;
  int pathCount, n;
  vector<int> size, parent, in, out, path, pathSize, pathPos, pathRoot;

  HeavyLight(vector<vector<int>> t)
      : tree(t), n(tree.size()), size(n), parent(n), in(n), out(n), path(n),
        pathSize(n), pathPos(n), pathRoot(n) {
    int time = 0;
    dfs(0, -1, time);
    buildPaths(0, newPath(0));
  }

  void dfs(int u, int p, int &k) {
    in[u] = k++, parent[u] = p, size[u] = 1;
    EACH(v, tree[u]) if (*v != p) dfs(*v, u, k), size[u] += size[*v];
    out[u] = k++;
  }

  int newPath(int u) {
    pathRoot[pathCount] = u;
    return pathCount++;
  }

  void buildPaths(int u, int pt) {
    path[u] = pt, pathPos[u] = pathSize[pt]++;
    EACH(v, tree[u])
    if (*v != parent[u])
        buildPaths(*v, 2 * size[*v] >= size[u] ? pt : newPath(*v));
  }

  bool isAncestor(int p, int ch) {
    return in[p] <= in[ch] && out[ch] <= out[p];
  }

  int lca(int a, int b) {
    for (int root; !isAncestor(root = pathRoot[path[a]], b); a = parent[root])
      ;
    for (int root; !isAncestor(root = pathRoot[path[b]], a); b = parent[root])
      ;
    return isAncestor(a, b) ? a : b;
  }
};

// Usage: http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=GRL_5_C
int main(void) {
  // ios::sync_with_stdio(false);

  int n;
  cin >> n;
  vector<vector<int>> tree(n);

  REP(i, n) {
    int k;
    cin >> k;
    REP(j, k) {
      int ch;
      cin >> ch;
      tree[i].push_back(ch);
    }
  }

  HeavyLight hl = HeavyLight(tree);

  int q;
  cin >> q;
  while (q--) {
    int u, v;
    cin >> u >> v;
    cout << hl.lca(u, v) << "\n";
  }

  return 0;
}