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;
}
}