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Validate Binary Search Tree

Link to leetcode problem definition

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.

Example 1:

    2
   / \
  1   3

Input: [2,1,3] Output: true

Example 2:

    5
   / \
  1   4
     / \
    3   6

Input: [5,1,4,null,null,3,6] Output: false

Explanation: The root node's value is 5 but its right child's value is 4.

class Solution {
    public boolean isValidBST(TreeNode root) {
        return isValid(root, null, null);
    }
    
    private boolean isValid(TreeNode node, Integer min, Integer max) {
        if(node == null) {
            return true;
        }
        
        if((min != null && node.val <= min) ||
           (max != null && node.val >= max)) {
            return false;
        }
        
        return isValid(node.left, min, node.val) 
               && isValid(node.right, node.val, max);
    }
}