[LeetCode] Problem 429 - N-ary Tree Level Order Traversal

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Given an n-ary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example, given a 3-ary tree:

E69bqA.png

We should return its level order traversal:

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[
     [1],
     [3,2,4],
     [5,6]
]

Note

  1. The depth of the tree is at most 1000.
  2. The total number of nodes is at most 5000.

Code

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public class Node {
    public int val;
    public List<Node> children;
    public Node(int _val, List<Node> _children) {
        val = _val;
        children = _children;
    }
}
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public List<List<Integer>> levelOrder(Node root) {
    List<List<Integer>> result = new ArrayList<>();

    if (root == null)
        return result;

    Queue<Node> queue = new LinkedList<>();
    queue.offer(root);

    while (!queue.isEmpty()) {
        int size = queue.size();
        List<Integer> nodes = new ArrayList<>();

        for (int i = 0; i < size; i++) {
            Node node = queue.poll();
            nodes.add(node.val);
            List<Node> children = node.children;

            if (children != null) {
                for (Node child : children)
                    queue.offer(child);
            }
        }

        result.add(nodes);
    }

    return result;
}