Validate Binary Search Tree

Problem

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.

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Idea

这个题一定要注意，判断bst并不是只检查以下当前节点和其左右子树是否正确。 应该确保每个节点都不大于当前子树的最大值和最小值。举个例子

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Solution

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