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? > read more on how binary tree is serialized on OJ.

注意本题在原有的函数上添加了默认参数，以便形成递归

class Solution {

public:

    bool isValidBST(TreeNode *root,int min = INT_MIN,int max = INT_MAX){

        if(!root) return true;

        if(root->val <= min || root->val>=max) return false;

        return isValidBST(root->left,min,root->val) && isValidBST(root->right,root->val, max);

    }

};