Validate Binary Search Tree

题目地址：
https://leetcode.com/problems/validate-binary-search-tree/#/description

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.

Example 1:

    2
   / \
  1   3

Binary tree [2,1,3], return true.

Example 2:

    1
   / \
  2   3

Binary tree [1,2,3], return false.

解题思路：
这道题主要就是用recursion的思想，首先判断当前node，然后再往左右个要一个结果再判断。

代码：

public boolean isValidBST(TreeNode root) {
    return helper(root, null, null);
}

private boolean helper(TreeNode root, Integer upper, Integer lower) {
    if(root == null){
        return true;
    }
    if((upper != null && root.val >= upper) || (lower != null && root.val <= lower)){
        return false;
    }
    boolean left = helper(root.left, root.val, lower);
    boolean right = helper(root.right, upper, root.val);
    return left && right;
}