ValidateBinarySearchTree.java

package com.dbc;

import java.util.Stack;

public class ValidateBinarySearchTree {
    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;
        }
    }

    public boolean isValidBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        long inorder = Long.MIN_VALUE;
        while (!stack.isEmpty() || root != null) {
            while (root != null) {
                stack.add(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.val <= inorder) return false;
            inorder = root.val;
            root = root.right;
        }
        return true;
    }
}