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.

Input:
    2
   /   1   3
Output: true

題意：

驗證BST

思路：

dfs

根據BST的性質： 左子樹<根<右子樹

要特別留心，對於input的root，沒有辦法給定一個確定的範圍。所以初始化為null

代碼：

 1 class Solution {
 2     public boolean isValidBST(TreeNode root) {
 3         return helper(root, null, null);
 4     }
 5     
 6     boolean helper(TreeNode root, Integer min, Integer max) {
 
 7         if (root == null)
 8             return true;
 9         
10         if ((min != null && root.val <= min) || (max != null && root.val >= max))
11             return false;
12         
13         return helper(root.left, min, root.val) && helper(root.right, root.val, max);
 
14     }
15 }

[leetcode]98. Validate Binary Search Tree驗證BST