429. N-ary Tree Level Order Traversal

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class Solution {
public List<List<Integer>> levelOrder(Node root) {
List<List<Integer>> resultList = new ArrayList<>();

Queue<Node> queue = new LinkedList<>();
if (root != null) {
queue.offer(root);
}

while (!queue.isEmpty()) {
List<Integer> numList = new ArrayList<>(queue.size());
for (int i = queue.size(); i > 0; i--) {
Node node = queue.poll();
numList.add(node.val);
if (node.children != null) {
for (Node child : node.children) {
queue.offer(child);
}
}
}
resultList.add(numList);
}

return resultList;
}
}

References

429. N-ary Tree Level Order Traversal