<p>Implement a function to check if a binary tree is a binary search tree.</p> <p><strong>Example 1:</strong></p> <pre> <strong>Input:</strong> 2 / \ 1 3 <strong>Output:</strong> true </pre> <p><strong>Example 2:</strong></p> <pre> <strong>Input:</strong> 5 / \ 1 4 / \ 3 6 <strong>Output:</strong> false <strong>Explanation:</strong> Input: [5,1,4,null,null,3,6]. the value of root node is 5, but its right child has value 4.</pre>