add leetcode problem-cn part2

算法题（国内版）/problem (English)/合法二叉搜索树(English) [legal-binary-search-tree-lcci].html

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+<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>