/**
 * 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;                     //递归到空节点，说明当前子树不包含p和q

        if (root == p || root == q) return root;    //递归找到p或q直接返回

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

        if (left && right) return root;             //p,q分别在左右子树，则返回当前节点
        if (left) return left;                      //p,q都在左子树，返回左节点
        else return right;
    }
};