617. Merge Two Binary Trees

Edited by Charles

Easy · Python · Binary Tree

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

Recursion:

if root1 and root2 are both None, then the new tree does not have the child at this position. if either root1 or root2 exists while the other is None, make the existing node as the new node.

Original study notes follow, including intermediate snippets and earlier attempts.

Recursion:

1. if root1 and root2 are both None, then the new tree does not have the child at this position.
2. if either root1 or root2 exists while the other is None, make the existing node as the new node.
3. if both two are valid, merge them by adding the values up and make a new node.

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 15, 2026

Python solution

# 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, root1: Optional[TreeNode], root2: Optional[TreeNode]) -> Optional[TreeNode]:
        if not root1 and not root2:
            return None
        if root1 and not root2:
            return root1
        if not root1 and root2:
            return root2

        root = TreeNode(root1.val + root2.val)
        root.left = self.mergeTrees(root1.left, root2.left)
        root.right = self.mergeTrees(root1.right, root2.right)

        return root

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