429. N-ary Tree Level Order Traversal

Given an n-ary tree, return the level order 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).

BFS Traversal to memorize:

List<Integer> values = new ArrayList<>();
Queue<Node> queue = new LinkedList<>();
queue.add(root);
while (!queue.isEmpty()) {
    Node nextNode = queue.remove();
    values.add(nextNode.val);
    for (Node child : nextNode.children) {
        queue.add(child);
    }
}

/*
// 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<List<Integer>> levelOrder(Node root) {
        List<List<Integer>> res = new ArrayList<>();
        Queue<Node> queue = new LinkedList<>();
        if (root ==null) return res;
        queue.add(root);
        while (!queue.isEmpty()){
            List<Integer> list = new ArrayList<>();
            int size = queue.size();
            for (int i=1; i<=size; i++){
                Node node = queue.poll();
                list.add(node.val);
                queue.addAll(node.children);
            }
            res.add(list);   
        }
        return res;
    }
}