LeetCode 590. N-ary Tree Postorder Traversal

Description

https://leetcode.com/problems/n-ary-tree-postorder-traversal/

Given an n-ary tree, return the postorder traversal of its nodes’ values.

Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value (See examples).

Follow up:

Recursive solution is trivial, could you do it iteratively?

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: [5,6,3,2,4,1]

Example 2:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [2,6,14,11,7,3,12,8,4,13,9,10,5,1]

Constraints:

  • The height of the n-ary tree is less than or equal to 1000
  • The total number of nodes is between [0, 10^4]

Explanation

Recursively visit nodes using post order.

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]:
        
        results = []
        
        self.helper(root, results)
        
        return results
        
        
    def helper(self, root, results):
        if not root:
            return
        
        for child in root.children:
            self.helper(child, results)
            
        
        results.append(root.val)
  • Time Complexity: O(N)
  • Space Complexity: O(N)

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