Leetcode #156: Binary Tree Upside Down
In this guide, we solve Leetcode #156 Binary Tree Upside Down in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
Given the root of a binary tree, turn the tree upside down and return the new root. You can turn a binary tree upside down with the following steps: The original left child becomes the new root.
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: Tree, Depth-First Search, Binary Tree
Intuition
We need to explore a structure deeply before backing up, which suits DFS.
DFS keeps local context on the call stack and is easy to implement recursively.
Approach
Define a recursive DFS that carries the necessary state.
Combine child results as the recursion unwinds.
Steps:
- Define a recursive DFS with state.
- Visit children and combine results.
- Return the final aggregation.
Example
Input: root = [1,2,3,4,5]
Output: [4,5,2,null,null,3,1]
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 upsideDownBinaryTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
if root is None or root.left is None:
return root
new_root = self.upsideDownBinaryTree(root.left)
root.left.right = root
root.left.left = root.right
root.left = None
root.right = None
return new_root
Complexity
The time complexity is O(V+E). The space complexity is O(V).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.