package org.example;

import java.util.ArrayDeque;

import java.util.ArrayList;

import java.util.List;

import java.util.Queue;

public class LevelOrder {

public List<List<Integer>> levelOrder(TreeNode root) {

Queue<TreeNode> queue = new ArrayDeque<>();

List<List<Integer>> list = new ArrayList<>();

if (root != null) {

queue.offer(root);

level(queue, list);

}

return list;

}

private void level(Queue<TreeNode> queue, List<List<Integer>> list) {

if (queue == null || queue.isEmpty()) {

return;

}

while (!queue.isEmpty()) {

List<Integer> l = new ArrayList<>();

int size = queue.size();

for (int i = 0; i < size; i++) {

TreeNode node = queue.poll();

l.add(node.val);

if (node.left != null) {

queue.offer(node.left);

}

if (node.right != null) {

queue.offer(node.right);

}

}

list.add(l);

}

}

}