617. Merge Two Binary Trees

1. Quetions:  617. Merge Two Binary Trees

url: https://leetcode.com/problems/merge-two-binary-trees/description/  

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

2. Solution

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution(object):

    def DFS(self, root1, root2):

        if root1 is None or root2 is None:
            return

        if root1 is not None and root2 is not None:
            root1.val += root2.val

        if root1.left is None:
            root1.left = root2.left
            root2.left = None
        else:
            self.DFS(root1.left, root2.left)

        if root1.right is None:
            root1.right = root2.right
            root2.right = None
        else:
            self.DFS(root1.right, root2.right)

    def mergeTrees(self, t1, t2):
        """
        :type t1: TreeNode
        :type t2: TreeNode
        :rtype: TreeNode
        """
        if t1 is None:
            return t2

        if t2 is None:
            return t1

        self.DFS(t1, t2)

        return t1