leetcode(617). Merge Two Binary Trees

problem

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.

分析

题目的意思就是把两个二叉树的对应的位置相加。所以这里我选择了递归的方法，代码如下：

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    def mergeTrees(self, t1, t2):        """        :type t1: TreeNode        :type t2: TreeNode        :rtype: TreeNode        """        if t1 != None and t2 != None:            t1.val += t2.val            t1.left = self.mergeTrees(t1.left, t2.left)            t1.right = self.mergeTrees(t1.right, t2.right)            return t1        elif t1 != None and t2 == None:            return t1        elif t1 == None and t2 != None:            return t2        else:            return None