LeetCode/problems/n-ary-tree-level-order-traversal.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 {
    private void levelOrder(ArrayList<List<Integer>> traversal, Node root, int level) {
        if (root == null) {
            return;
        }

        if (level >= traversal.size()) {
            traversal.add(new ArrayList<Integer>());
        }
        traversal.get(level).add(root.val);

        for (var child : root.children) {
            levelOrder(traversal, child, level + 1);
        }
    }

    public List<List<Integer>> levelOrder(Node root) {
        ArrayList<List<Integer>> result = new ArrayList<>();
        levelOrder(result, root, 0);
        return result;
    }
}