0236.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
  TreeNode *lowestCommonAncestor(TreeNode *root, TreeNode *p, TreeNode *q) {
    // return func1(root, p, q);
    return func2(root, p, q);
  }

  TreeNode *func1(TreeNode *root, TreeNode *p, TreeNode *q) {
    vector<TreeNode *> p_anc;
    vector<TreeNode *> q_anc;
    findAncestor(root, p, p_anc);
    findAncestor(root, q, q_anc);

    int p_size = p_anc.size();
    int q_size = q_anc.size();
    int size = min(p_size, q_size);
    TreeNode *ancestor = NULL;
    for (int i = 0; i < size; i++) {
      if (p_anc[i] == q_anc[i]) {
        ancestor = p_anc[i];
      } else {
        break;
      }
    }
    return ancestor;
  }
  bool findAncestor(TreeNode *root, TreeNode *node,
                    vector<TreeNode *> &ancestor) {
    if (root == NULL) {
      return false;
    }

    ancestor.emplace_back(root);
    if (root == node) {
      return true;
    }

    if (findAncestor(root->left, node, ancestor)) {
      return true;
    }
    if (findAncestor(root->right, node, ancestor)) {
      return true;
    }

    ancestor.pop_back();
    return false;
  }

  TreeNode *func2(TreeNode *root, TreeNode *p, TreeNode *q) {
    if (root == NULL || root == p || root == q) {
      return root;
    }

    TreeNode *left = func2(root->left, p, q);
    TreeNode *right = func2(root->right, p, q);

    if (left != NULL && right != NULL) {
      return root;
    }

    return left != NULL ? left : right;
  }
};