LevelOrderTraversal.java

import java.util.*;

class TreeNode {
    int val;
    TreeNode left, right;
    TreeNode(int x) { val = x; }
}

public class LevelOrderTraversal {

    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();

        if (root == null) return result;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root); // Start with root

        while (!queue.isEmpty()) {
            int levelSize = queue.size(); // Number of nodes at current level
            List<Integer> level = new ArrayList<>();

            for (int i = 0; i < levelSize; i++) {
                TreeNode current = queue.poll(); // Remove from queue
                level.add(current.val);

                if (current.left != null)
                    queue.offer(current.left);

                if (current.right != null)
                    queue.offer(current.right);
            }

            result.add(level); // Add this level to final result
        }

        return result;
    }
}