Leetcode #590: N-ary Tree Postorder Traversal
In this guide, we solve Leetcode #590 N-ary Tree Postorder Traversal 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 an n-ary tree, return the postorder traversal of its nodes' values. Nary-Tree input serialization is represented in their level order traversal.
Quick Facts
- Difficulty: Easy
- Premium: No
- Tags: Stack, Tree, Depth-First Search
Intuition
The problem has a natural nested or last-in-first-out structure.
A stack lets us resolve matches in the correct order as we scan.
Approach
Push items as they appear and pop when you can finalize a decision.
The stack captures the unresolved part of the input.
Steps:
- Push elements as you scan.
- Pop when a rule or match is satisfied.
- Use the stack to compute results.
Example
Input: root = [1,null,3,2,4,null,5,6]
Output: [5,6,3,2,4,1]
Python Solution
"""
# Definition for a Node.
class Node:
def __init__(self, val=None, children=None):
self.val = val
self.children = children
"""
class Solution:
def postorder(self, root: 'Node') -> List[int]:
def dfs(root):
if root is None:
return
for child in root.children:
dfs(child)
ans.append(root.val)
ans = []
dfs(root)
return ans
Complexity
The time complexity is , and the space complexity is . The space complexity is .
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.