# Time:  O(n)
# Space: O(h)

# Given the root of a binary tree, then value v and depth d,
# you need to add a row of nodes with value v at the given depth d. The root node is at depth 1.
#
# The adding rule is: given a positive integer depth d,
# for each NOT null tree nodes N in depth d-1, create two tree nodes
# with value v as N's left subtree root and right subtree root.
# And N's original left subtree should be the left subtree of the new left subtree root,
# its original right subtree should be the right subtree of the new right subtree root.
# If depth d is 1 that means there is no depth d-1 at all,
# then create a tree node with value v as the new root of the whole original tree,
# and the original tree is the new root's left subtree.
#
# Example 1:
# Input: 
# A binary tree as following:
#        4
#      /   \
#     2     6
#    / \   / 
#   3   1 5   
#
# v = 1
#
# d = 2
#
# Output: 
#        4
#       / \
#      1   1
#     /     \
#    2       6
#   / \     / 
#  3   1   5   
#
# Example 2:
# Input: 
# A binary tree as following:
#       4
#      /   
#     2    
#    / \   
#   3   1    
#
# v = 1
#
# d = 3
#
# Output: 
#       4
#      /   
#     2
#    / \    
#   1   1
#  /     \  
# 3       1
# Note:
# 1. The given d is in range [1, maximum depth of the given tree + 1].
# 2. The given binary tree has at least one tree node.

# 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 addOneRow(self, root, v, d):
        """
        :type root: TreeNode
        :type v: int
        :type d: int
        :rtype: TreeNode
        """
        if d in (0, 1):
            node = TreeNode(v)
            if d == 1:
                node.left = root
            else:
                node.right = root
            return node
        if root and d >= 2:
            root.left  = self.addOneRow(root.left,  v, d-1 if d > 2 else 1)
            root.right = self.addOneRow(root.right, v, d-1 if d > 2 else 0)
        return root

Create add-one-row-to-tree.py