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.
  

题目大意：判断给定的树是不是二叉搜索树

题目难度：Medium

import java.util.Stack;
/**
 * Created by gzdaijie on 16/6/5
 * 对于二叉搜索树,中序遍历的结果是递增的
 */
public class Solution {
    public boolean isValidBST(TreeNode root) {
        long pre = Integer.MIN_VALUE;
        Stack<TreeNode> stack = new Stack<>();

        pre = pre - 1;
        while (true) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            if (stack.isEmpty()) break;
            TreeNode tmp = stack.pop();

            if (tmp.val <= pre) return false;
            if (tmp.right != null) root = tmp.right;

            pre = tmp.val;
        }
        return true;
    }
}