Merge Two Binary Trees Python Solution

Tested Python solution for LeetCode 617 with 12 pytest cases. Generate a practice environment with lcpy.

LeetCode 617, Easy. Topics: Tree, Depth-First Search, Breadth-First Search, Binary Tree. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 12 parametrized pytest cases, and a playground notebook:

lcpy gen -n 617   # by problem number
lcpy gen -s merge_two_binary_trees   # by problem name

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Problem

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.

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Examples

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

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

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Constraints

• The number of nodes in both trees is in the range [0, 2000].
• -10^4 <= Node.val <= 10^4

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

from leetcode_py import TreeNode


class Solution:
    # Time: O(n) where n is the smaller tree's node count
    # Space: O(h)
    def merge_trees(
        self, root1: TreeNode[int] | None, root2: TreeNode[int] | None
    ) -> TreeNode[int] | None:
        if root1 is None:
            return root2
        if root2 is None:
            return root1
        root1.val += root2.val
        root1.left = self.merge_trees(root1.left, root2.left)
        root1.right = self.merge_trees(root1.right, root2.right)
        return root1

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Complexity

Time	Space
O(n) where n is the smaller tree’s node count	O(h)

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Tags

NeetCode All.