（难度Medium）Problem 98. Validate Binary Search Tree（判断二分查找树是否合法）

### 链接

Validate Binary Search Tree - LeetCode

Medium

### 题干

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.

### 样例

Example 1:

    2
/ \
1   3

**Input:** [2,1,3]
**Output:** true

Example 2:

    5
/ \
1   4
/ \
3   6

**Input:** [5,1,4,null,null,3,6]
**Output:** false
**Explanation:** The root node’s value is 5 but its right child’s value is 4.