CODE 35: Validate Binary Search Tree leetcode

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.

【CODE 35: Validate Binary Search Tree】
confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.

OJ's Binary Tree Serialization: The serialization of a binary tree follows a level order traversal, where '#' signifies a path terminator where no node exists below.
Here's an example:

1 / \ 23 / 4 \ 5

The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".

public boolean isValidBST(TreeNode root) { // Start typing your Java solution below // DO NOT write main() function if (null == root) { return true; } Stack stack = new Stack(); TreeNode pre = new TreeNode(Integer.MIN_VALUE); TreeNode now = root; while (!stack.isEmpty() || null != now) { if (null == now) { now = stack.pop(); if (pre.val >= now.val) { return false; } pre = now; now = now.right; } else { stack.push(now); now = now.left; } } return true; }