LeetCode 102: Binary Tree Level Order Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        if (root == null) {
            return Collections.emptyList();
        }
        
        List<List<Integer>> result = new ArrayList<>();
        List<Integer> row = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        int level = 1;
        int future = 0;
        
        while (!queue.isEmpty()) {
            TreeNode current = queue.poll();
            if (current.left != null) {
                queue.offer(current.left);
                future++;
            }
            
            if (current.right != null) {
                queue.offer(current.right);
                future++;
            }
            
            row.add(current.val);
            level--;
            
            if (level == 0) {
                level = future;
                future = 0;
                result.add(new ArrayList<>(row));
                row.clear();
            }
        }
        
        return result;
    }
}