/**

 * 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 == NULL)

            return NULL;

        if (root->val > p->val && root->val > q->val)

            return lowestCommonAncestor(root->left,p,q);

        if (root->val < p->val && root->val < q->val)

            return lowestCommonAncestor(root->right,p,q);

        return root;   //出现了一大一小，就可以返回root了

    }

};