[Leetcode] 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.

Analysis:

class Solution {
public:

    bool helper(TreeNode *root, int minVal, int maxVal) {
        if (root == NULL)
            return true;
        else
            return root->val < maxVal && 
            root->val > minVal &&
            helper(root->left, minVal, root->val) &&
            helper(root->right, root->val, maxVal);
        
    }

    bool isValidBST(TreeNode *root) {
        return helper(root, INT_MIN, INT_MAX);
    }
};