Problem Description

Given the root of a binary tree, determine if it is a valid binary search tree (BST).

A valid 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.

Examples

Example 1:

Input: root = [2,1,3]
Output: true

Example 2:

Input: root = [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.

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• 231 <= Node.val <= 231 - 1

解析

題目給定一個二元樹的根結點 root

想要判斷這個由 root 所形成的樹是不是一棵二元搜尋樹

如果 root 所形成的樹是一個二元搜尋樹有以下特性