[LeetCode&Python] Problem 617. Merge Two Binary Trees

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

class Solution:
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if t1==None and t2!=None:
            return t2
        elif t1!=None and t2==None:
            return t1
        elif t1==None and t2==None:
            return None
        else:
            def submergetree(r1,r2):
                if r1!=None and r2==None:
                    return r1
                elif r1==None and r2!=None:
                    return r2
                elif r1==None and r2==None:
                    return None
                else:
                    r1.val+=r2.val
                    r1.left=submergetree(r1.left,r2.left)
                    r1.right=submergetree(r1.right,r2.right)
                    return r1
            
            return submergetree(t1,t2)