Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

• The left subtree of a node contains only nodes with keys less than the node's key.
• The right subtree of a node contains only nodes with keys greater than the node's key.
• Both the left and right subtrees must also be binary search trees.

confused what "{1,#,2,3}" means? 

从根节点開始往以下遍历，然后不断的更新上界和下界就可以。

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:	bool check(TreeNode *node,int lv,int rv){		if(node==NULL) return true;		return lv
     
      val&&node->val
      
       left,lv,node->val)&&check(node->right,node->val,rv);	}    bool isValidBST(TreeNode *root) {    	return check(root,INT_MIN,INT_MAX);        }};