Validate Binary Search Tree

题目：

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.
A single node tree is a BST

分析：

解法1是老方法，逻辑已经忘记了...

还有一种用result type的方法可以做好这道题

解法1：

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: True if the binary tree is BST, or false
     */
    public boolean ifReachedBottomLeft = false;
    int prev = Integer.MIN_VALUE;

    public boolean isValidBST(TreeNode root) {
        // write your code here
        if (root == null) {
            return true;
        }

        if (!isValidBST(root.left) ) {
            return false;
        } 

        // do something
        if (ifReachedBottomLeft == true && prev >= root.val) {
            return false;
        }

        ifReachedBottomLeft = true;
        prev = root.val;


        if (!isValidBST(root.right) ) {
            return false;
        }

        return true;

    }
}