LeetCode - Binary Tree Level Order Traversal II

/**
* Definition for binary tree
* 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<Integer>> list = new ArrayList<>();
List<List<Integer>> trlist = new ArrayList<>();
bfs(0, root, list);
for(int i=list.size()-1; i>=0; i--) {
trlist.add(list.get(i));
}
return trlist;

}

public void bfs(int pos, TreeNode root, List<List<Integer>> list) {
if(root==null) return;
if(list.size() > pos) {
list.get(pos).add(root.val);
}
else {
List<Integer> l = new ArrayList<>();
l.add(root.val);
list.add(l);
}
bfs(pos+1, root.left, list);
bfs(pos+1, root.right, list);
}

}