617. Merge Two Binary Trees (Easy)

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.

思路：

1.二叉树的遍历： 遍历每个节点，若有重叠（两个二叉树节点都不为空），新节点值即为两者之和；若不重叠（只有一个节点为空），新节点值即为那个不为空的值；全为空则到达底部返回跳出；

2.深度优先遍历（纵向）：列表

   广度优先遍历（横向）：迭代

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