/**
 * 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;
        TreeNode* l=lowestCommonAncestor(root->left, p, q);
        TreeNode* r=lowestCommonAncestor(root->right, p, q);
        
        if(!l) return r;
        else if(!r) return l;
        else return root;
    }
};