Validate Binary Search Tree

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.

class Solution {  public:      bool isValidBST(TreeNode *root) {                    return checkValidBST(root, numeric_limits<int>::min(), numeric_limits<int>::max());      }            bool checkValidBST(TreeNode *root, int min_val, int max_val) {          if (root == NULL) {              return true;          }                    bool valid = false;          if (root->val >= min_val && root->val <= max_val) {              valid = checkValidBST(root->left, min_val, root->val - 1);              if (valid) {                  valid = checkValidBST(root->right, root->val + 1, max_val);              }          }                    return valid;      }  };