0589-n-ary-tree-preorder-traversal.rb

# frozen_string_literal: true

# 589. N-ary Tree Preorder Traversal
# https://leetcode.com/problems/n-ary-tree-preorder-traversal

=begin

Given the root of an n-ary tree, return the preorder 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)

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

### 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: [1,2,3,6,7,11,14,4,8,12,5,9,13,10]

### Constraints:
* The number of nodes in the tree is in the range [0, 104].
* 0 <= Node.val <= 104
* The height of the n-ary tree is less than or equal to 1000.

=end

# Definition for a Node.
# class Node
#     attr_accessor :val, :children
#     def initialize(val)
#         @val = val
#         @children = []
#     end
# end

# Runtime: 112 ms
# Memory: 211.7 MB
# @param {Node} root
# @return {List[int]}
def preorder(root)
  loop.reduce([[], root ? [root] : []]) do | (r, s) |
    s.empty? ? (break r) : [
      r << (root = s.pop).val, s.push(*root.children.reverse_each)
    ]
  end
end