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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).

Example 1:

img

Input: root = [1,null,3,2,4,null,5,6]
Output: [[1],[3,2,4],[5,6]]

Example 2:

img

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: [[1],[2,3,4,5],[6,7,8,9,10],[11,12,13],[14]]

Constraints:

  • The height of the n-ary tree is less than or equal to 1000
  • The total number of nodes is between [0, 104]

Solution:

/*
// 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>> result = new ArrayList<>();
        if (root == null){
            return result;
        }

        Deque<Node> queue = new ArrayDeque<>();

        queue.offerLast(root);
        int level = 0;
        while(!queue.isEmpty()){
            int curLevelSize = queue.size();
            if (result.size() <= level){
                result.add(new ArrayList<>());
            }

            for (int i = 0; i < curLevelSize; i++){
                Node cur = queue.pollFirst();
                result.get(level).add(cur.val);

                for (Node child : cur.children){
                    queue.offerLast(child);
                }
            }
            level++;
        }

        return result;
    }
}

// TC: O(n)
// SC: O(n)