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