N-ary Tree Preorder Traversal

Easy

Given an n-ary tree, return thepreordertraversal of its nodes' values.

For example, given a3-arytree:

Return its preorder traversal as:[1,3,5,6,2,4].

Note:

Recursive solution is trivial, could you do it iteratively?

Solution

// Definition for a Node.

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

    public Node() {}

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

DFS - Recursive

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

        return res;
    }

    private void preorderHelper(Node root, List<Integer> res) {
        if (root == null) {
            return;
        }

        res.add(root.val);

        for (Node node: root.children) {
            preorderHelper(node, res);
        }
    }
}

DFS - Iterative

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

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    public List<Integer> preorder(Node root) {

        List<Integer> res = new ArrayList<>();
        if (root == null) {
            return res;
        }
        Deque<Node> stack = new ArrayDeque<>();

        stack.push(root);

        while (!stack.isEmpty()) {
            Node node = stack.pop();
            res.add(node.val);
            if (node.children != null) {
                Collections.reverse(node.children);
                for (Node c: node.children) {
                    stack.push(c);
                }
            }
        }
        return res;
    }
}

Reference

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