617 Merge Two Binary Trees.py

#!/usr/bin/python3
"""
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.
"""


# 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 not t1 and not t2:
            return

        node = TreeNode(0)
        node.val += t1 and t1.val or 0
        node.val += t2 and t2.val or 0
        node.left = self.mergeTrees(t1 and t1.left, t2 and t2.left)
        node.right = self.mergeTrees(t1 and t1.right, t2 and t2.right)
        return node