429. N-ary Tree Level Order Trav

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).

Solution：

思路： Simple BFS

Time Complexity: O(N) Space Complexity: O(N)

Solution Code：

class Solution {
    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()) {
            List<Integer> curLevelResult = new ArrayList<>();
            
            int size = queue.size();
            for (int i = 0; i < size; i++) {
                Node curNode = queue.poll();
                curLevelResult.add(curNode.val);
                for (Node child : curNode.children) {
                    queue.offer(child);
                }
            }
            result.add(curLevelResult);
        }
        return result;
    }
}