/**
 * 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) {
        if (!root) return NULL;
        if (root == p || root == q) return root;//恰好遇到p或者q
        auto left = lowestCommonAncestor (root->left, p, q);
        auto right = lowestCommonAncestor (root->right, p, q);
        if (left && right) return root;
        if (left) return left;
        return right;
    }
};