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

# Given two binary trees and imagine that 
# when you put one of them to cover the other,
# some nodes of the two trees are overlapped
# while the others are not.
#
# You need to merge them into a new binary tree.
# The merge rule is that if two nodes overlap,
# then sum node values up as the new value of the merged node.
# Otherwise, the NOT null node will be used as the node of new tree.
#
# Example 1:
# Input: 
# 	Tree 1                     Tree 2                  
#           1                         2                             
#          / \                       / \                            
#         3   2                     1   3                        
#       /                           \   \                      
#       5                             4   7                  
# Output: 
# Merged tree:
# 	     3
# 	    / \
# 	   4   5
# 	  / \   \ 
# 	 5   4   7
	 
# Note: The merging process must start from the root nodes of both trees.

# 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 mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if t1 is None:
            return t2
        if t2 is None:
            return t1
        t1.val += t2.val
        t1.left = mergeTrees(t1.left, t2.left)
        t1.right = mergeTrees(t1.right, t2.right)
        return t1

Create merge-two-binary-trees.py