589. N-ary Tree Preorder Traversal

Problem

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, 10^4].

• 0 <= Node.val <= 10^4

• The height of the n-ary tree is less than or equal to 1000.

  Follow up: Recursive solution is trivial, could you do it iteratively?

Solution (Java)

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    public List<Integer> preorder(Node root) {
        List<Integer> res = new ArrayList<>();
        preorderHelper(res, root);
        return res;
    }

    private void preorderHelper(List<Integer> res, Node root) {
        if (root == null) {
            return;
        }
        res.add(root.val);
        for (Node node : root.children) {
            preorderHelper(res, node);
        }
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).