leetcode 107 Binary Tree Level Order Traversal II ----- java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
            

        List list = new ArrayList<List<Integer>>();

        if( root == null)
            return list;

        Queue<TreeNode> tree = new LinkedList<TreeNode>();


        tree.add(root);
        while( !tree.isEmpty() ){
            List ans = new ArrayList<Integer>();
            int size = tree.size();
            for( int i = 0;i<size;i++){
                TreeNode node = tree.poll();
                ans.add(node.val);
                if( node.left != null )
                    tree.add(node.left);
                if( node.right != null)
                    tree.add(node.right);
            }
            if( !ans.isEmpty() )
                list.add(0,ans);



        }



        return list;


    }
}