Write an algorithm to find the "next" node (i.e., in-order successor) of a given node in a binary search tree.

Return null if there's no "next" node for the given node.

Example 1:

Input: root = [2,1,3], p = 1

  2
 / \
1   3

Output: 2

Example 2:

Input: root = [5,3,6,2,4,null,null,1], p = 6

      5
     / \
    3   6
   / \
  2   4
 /   
1

Output: null