Coding-Exercise/Tree/98.validate-binary-search-tree.java at master · Fettes/Coding-Exercise · GitHub

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/*
 * @lc app=leetcode id=98 lang=java
 *
 * [98] Validate Binary Search Tree
 */

// @lc code=start
/**
 * 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);
    }

    public boolean helper(TreeNode root, Integer low, Integer high) {
        if (root == null) {
            return true;
        }
        if (low != null && root.val <= low) {
            return false;
        }
        if (high != null && root.val >= high) {
            return false;
        }

        return helper(root.left, low, root.val) && helper(root.right, root.val, high);
    }
}
// @lc code=end