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Validate Binary Search Tree

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Description

https://leetcode.com/problems/validate-binary-search-tree/

Solution

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/**
 * 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) {
        return helper(LONG_MIN, LONG_MAX, root);
    }
    
    bool helper(long lowerBound, long upperBound, TreeNode* root) {
        if (root == NULL) return true;
        if (root->val <= lowerBound 
            || root->val >= upperBound) {
            return false;
        }
        
        return helper(lowerBound, root->val, root->left) 
                    && helper(root->val, upperBound, root->right);
    }
    
};