Inorder Successor in BST

inorder_successor.py

"""
Description:
Given a binary search tree and a node in it, find the in-order successor of that node
in the BST. If the given node has no in-order successor in the tree, return null.

Examples:
Input: root = [2,1,3], p = 1
  2
 / \
1   3
Output: 2

Input: root = [5,3,6,2,4,null,null,1], p = 6
      5
     / \
    3   6
   / \
  2   4
 /   
1
Output: null
"""

class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

def inorderSuccessor(root, p):
    """
    :type root: TreeNode
    :type p: TreeNode
    :rtype: TreeNode
    """
    if not root or not p: return None
    if root.val > p.val:
        return inorderSuccessor(root.left, p) or root
    return inorderSuccessor(root.right, p)