ValidateBinarySearchTree.java

import java.util.Stack;

/**
 * Given a binary tree, determine if it is a valid binary search tree (BST).
 * <p>
 * Assume a BST is defined as follows:
 * <p>
 * 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
 * Binary tree [2,1,3], return true.
 * Example 2:
 * 1
 * / \
 * 2   3
 * Binary tree [1,2,3], return false.
 * <p>
 * Accepted.
 */
public class ValidateBinarySearchTree {

    public boolean isValidBST(TreeNode root) {
        if (root == null) {
            return true;
        }

        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (pre != null && root.val <= pre.val) {
                return false;
            }
            pre = root;
            root = root.right;
        }

        return true;
    }

    public static class TreeNode {

        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }

        @Override
        public boolean equals(Object obj) {
            if (obj instanceof TreeNode) {
                TreeNode node = (TreeNode) obj;
                if (this.val != node.val) {
                    return false;
                }
                if (this.left == null) {
                    if (this.right == null) {
                        return node.left == null && node.right == null;
                    }
                    return this.right.equals(node.right);
                }
                if (this.right == null) {
                    return node.right == null;
                }
                return this.left.equals(node.left) && this.right.equals(node.right);
            }
            return false;
        }

    }

}