Binary Tree Level Order Traversal II

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    List<List<Integer>> list = new ArrayList<>();
    List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        if(root == null) return res;
        dfs(root, 0);
        Collections.sort(list, new Comparator<List<Integer>>(){
            public int compare(List<Integer> o1, List<Integer> o2){
                return -(o1.get(1) - o2.get(1));
            } 
        });
        //System.out.println(list);
        List<Integer> temp = new ArrayList<>();
        int prev = list.get(0).get(1);
        for(int i = 0; i<list.size(); i++){
            if(list.get(i).get(1) == prev){  
                temp.add(list.get(i).get(0));
            }
            else{
                prev = list.get(i).get(1);
                //System.out.println(temp);
                res.add(new ArrayList<Integer>(temp));
                temp = new ArrayList<Integer>();
                temp.add(list.get(i).get(0));
            }
        }
        if(list.size() > 0){
            temp = new ArrayList<>();
            temp.add(list.get(list.size()-1).get(0));
            res.add(new ArrayList<Integer>(temp));
        }
        
        return res;
    }
    public void dfs(TreeNode root, int depth){
        if(root == null){
            return ;
        }
        List<Integer> temp = new ArrayList<>();
        temp.add(root.val);
        temp.add(depth);
        list.add(new ArrayList<Integer>(temp));
        dfs(root.left, depth+1);
        dfs(root.right, depth+1);
    }
}