0098 - Validate Binary Search Tree

0098 - Validate Binary Search Tree

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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.

Examples

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

Input: root = [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.

Constraints

The number of nodes in the tree is in the range [1, 104]. -231 <= Node.val <= 231 - 1

Java Solution

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        return helper(root, null, null);
    }
    
    private boolean helper(TreeNode root, Integer low, Integer high) {
        if(root == null) return true;
        
        if((low != null && root.val <= low) || (high != null && root.val >= high)) return false;
        
        return helper(root.right, root.val, high) && helper(root.left, low, root.val);
    }
}