Implement a function to check if a binary tree is a binary search tree.

Example 1:

Input:

    2

   / \

  1   3

Output: true

Example 2:

Input:

    5

   / \

  1   4

     / \

    3   6

Output: false

Explanation: Input: [5,1,4,null,null,3,6].

     the value of root node is 5, but its right child has value 4.