/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
// Approach #1 - BFS - T O(#OfNodes).
public class Solution {
    /**
     * @param root: A Tree
     * @return: Level order a list of lists of integer
     */
    public List<List<Integer>> levelOrder(TreeNode root) {
        // write your code here
        if (root == null) {
            return new LinkedList<>();
        }

        result = new LinkedList<>();
        bfs(root);
        return result;
    }

    private List<List<Integer>> result;

    private void bfs(TreeNode root) {
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            int size = queue.size();
            result.add(new LinkedList<>());
            while (size-- > 0) {
                TreeNode node = queue.poll();
                result.get(result.size() - 1).add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
        }
    }
}