leetcode-problemset/leetcode-cn/problem (English)/合法二叉搜索树(English) [legal-binary-search-tree-lcci].html

<p>Implement a function to check if a binary tree is a binary search tree.</p>



<p><strong>Example&nbsp;1:</strong></p>



<pre>

<strong>Input:</strong>

    2

   / \

  1   3

<strong>Output:</strong> true

</pre>



<p><strong>Example&nbsp;2:</strong></p>



<pre>

<strong>Input:</strong>

    5

   / \

  1   4

&nbsp;    / \

&nbsp;   3   6

<strong>Output:</strong> false

<strong>Explanation:</strong> Input: [5,1,4,null,null,3,6].

&nbsp;    the value of root node is 5, but its right child has value 4.</pre>