# 617. Merge Two Binary Trees 合并二叉树

@TOC

## # 题目描述

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
``````

## # 解题方法

### # 递归

``````class Solution:
def mergeTrees(self, t1, t2):
"""
:type t1: TreeNode
:type t2: TreeNode
:rtype: TreeNode
"""
if t1 and t2:
newT = TreeNode(t1.val +  t2.val)
newT.left = self.mergeTrees(t1.left, t2.left)
newT.right = self.mergeTrees(t1.right, t2.right)
return newT
else:
return t1 or t2
``````

``````# 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 not t2:
return t1
if not t1:
return t2
newT = TreeNode(t1.val + t2.val)
newT.left = self.mergeTrees(t1.left, t2.left)
newT.right = self.mergeTrees(t1.right, t2.right)
return newT
``````

## # 日期

2018 年 1 月 13 日 2018 年 11 月 3 日 —— 雾霾的周六