【Tree】617. Merge Two Binary Tree

问题描述：

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

Note:The merging process must start from the root nodes of both trees.

解题思路：

从根节点开始，如果其中一棵子树为空，直接拷贝另一棵子树到合并的树上。如果都不为空，直接加和当前节点的值，然后递归左右子树。

合并树可以新建一棵树，也可以使用两棵子树中的一个。代码采取前者。

Python3 实现：

# 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: TreeNode, t2: TreeNode) -> TreeNode:
        if t1 == None and t2 == None:
            return None 
        elif t1 == None:
            return t2
        elif t2 == None:
            return t1
        t = TreeNode(t1.val + t2.val)
        t.left = self.mergeTrees(t1.left, t2.left)
        t.right = self.mergeTrees(t1.right, t2.right)
        return t