98. Validate Binary Search Tree

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.

Example 1:

    2   / \  1   3

Binary tree [2,1,3], return true.

Example 2:

    1   / \  2   3

Binary tree [1,2,3], return false.

参照中序遍历模板JAVA 二叉树遍历，增加一个prev的节点。代码如下：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public boolean isValidBST(TreeNode root) {        Stack<TreeNode> stack = new Stack<TreeNode>();        TreeNode prev = null;        while (root != null || stack.size() > 0) {            while (root != null) {                stack.push(root);                root = root.left;            }            root = stack.pop();            if (prev != null && prev.val >= root.val) {                return false;            }            prev = root;            root = root.right;        }        return true;    }}