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.