合并两棵二叉树

Merge Two Binary Trees

• 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. 遇到树问题，首先想到递归

2. 将t2的val加到t1，返回当前处理的t1结点

3. 如果t1为null，把引用指向t2

4. 需要注意处理null的问题

代码

# Definition for a binary tree node.
class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution(object):
    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if t1 is not None and t2 is not None:
            t1.val += t2.val
            t1.left = self.mergeTrees(t1.left, t2.left)
            t1.right = self.mergeTrees(t1.right, t2.right)
        elif t1 is None and t2 is not None:
            t1 = t2
        return t1

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