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Merge Two Binary Trees

Problem Statement​

You are given two binary trees root1 and root2.

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 the two trees 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 the new tree.

Return the merged tree.

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

Leetcode Link

Example 1:​

Input: root1 = [1,3,2,5], root2 = [2,1,3,null,4,null,7]
Output: [3,4,5,5,4,null,7]

Example 2:​

Input: root1 = [1], root2 = [1,2]
Output: [2,2]

Constraints:​

  • The number of nodes in both trees is in the range [0, 2000].
  • -104 <= Node.val <= 104

Code​

Python Code

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def mergeTrees(self, t1, t2):
        if not t1 and not t2: return None
        ans = TreeNode((t1.val if t1 else 0) + (t2.val if t2 else 0))
        ans.left = self.mergeTrees(t1 and t1.left, t2 and t2.left)
        ans.right = self.mergeTrees(t1 and t1.right, t2 and t2.right)
        return ans