236

/**
 * 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* lca;
    TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
        lca = NULL;
        bool found = false;
        dfs(root, p, q, found);
        return lca;
    }

    pair<bool, bool> dfs(TreeNode* root, TreeNode*p, TreeNode* q, bool &found){
        if(found) return {true, true};
        if(!root) return {false, false};

        auto left = dfs(root->left, p, q, found);
        auto right = dfs(root->right, p, q, found);

        pair<bool, bool> seen;
        seen.first = (left.first || right.first);
        seen.second = (left.second || right.second);

        if(root->val == p->val) seen.first = true;
        if(root->val == q->val) seen.second = true;

        if(seen.first && seen.second && !found){
            found = true;
            lca = root;
        }
        return seen;
    }

};