617. Merge Two Binary Trees | June 11,2017

[HanchengZhao]

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:

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.

[HanchengZhao]

We can use a recursive solution to store each tree node value layer by layer, merge 2 trees together:

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

[YeWang0]

Is there an iterative way to solve it?