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>