/**

 * 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:

    bool isValidBST(TreeNode* root) {

        vector<int> res;

        int flag = ;

        long num = LONG_MIN;

        dfs(root,num,flag);

        return flag;

    }

    void dfs(TreeNode* root,long& maxnum,int& flag){

        if(root == NULL) return;

        if(root->left) dfs(root->left,maxnum,flag);

        if(root->val > maxnum){

            maxnum = root->val;

        }

        else{

            flag = ;

        }

        if(root->right) dfs(root->right,maxnum,flag);

    }

};