Solution.java

// Stepsort · Binary Tree Level Order
// Category: LeetCode Patterns
// Animated walkthrough: https://stepsort.prakashraj.me/algorithm/binary-tree-level-order

import java.util.*;

public class Main {
    static class TreeNode {
        int val;
        TreeNode left, right;
        TreeNode(int v) { val = v; }
    }

    public static List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> levels = new ArrayList<>();
        if (root == null) return levels;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> level = new ArrayList<>();
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                level.add(node.val);
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            levels.add(level);
        }
        return levels;
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(3);
        root.left = new TreeNode(9);
        root.right = new TreeNode(20);
        root.right.left = new TreeNode(15);
        root.right.right = new TreeNode(7);
        System.out.println(levelOrder(root));
    }
}