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:
            return t2
        if t2==None:
            return t1
        t1.val=t1.val+t2.val
        t1.left = self.mergeTrees(t1.left,t2.left)
        t1.right = self.mergeTrees(t1.right,t2.right)
        return t1
 

None,self.

一個類，他用init初始化的物件，是這個類的屬性，那下面的各個函式都是類的方法，如果某一個函式，它要呼叫類的屬性，前面都要用self，如果他要呼叫類的方法，包括呼叫它自身也是要前面加self.