[LeetCode]Binary Tree Level Order Traversal II

Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
For example: Given binary tree  {3,9,20,#,#,15,7} ,

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

confused what  "{1,#,2,3}"  means?
Tree Breadth-first Search

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
	List<List<Integer>> res = new ArrayList<>();
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        levelOrderBottom(root,0);
        Collections.reverse(res);
        return res;
    }
    
    private void levelOrderBottom(TreeNode root, int level){
    	if(root == null) return;
    	if(level==res.size()){
    		List<Integer> list = new ArrayList<>();
    		res.add(list);
    	}
    	res.get(level).add(root.val);
    	levelOrderBottom(root.left,level+1);
    	levelOrderBottom(root.right,level+1);
    }
}