Inorder Successor in BST

recursive:

If target node is smaller than its parent, just choose left child and do the recursion.

If parent is greater than and equal to target node, choose the right child

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def inorderSuccessor(self, root, p):
        """
        :type root: TreeNode
        :type p: TreeNode
        :rtype: TreeNode
        """

        if not root:
            return root


        if root.val > p.val:
            left = self.inorderSuccessor(root.left, p)
            if left == None:
                return root
            else:
                return left
        else:
            return self.inorderSuccessor(root.right, p)

iteration:

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def inorderSuccessor(self, root, p):
        """
        :type root: TreeNode
        :type p: TreeNode
        :rtype: TreeNode
        """

        if not root:
            return root

        successor = None
        while root:
            if root.val > p.val:
                successor = root
                root = root.left
            else:
                root = root.right

        return successor