LeetCode 107: 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? > read more on how binary tree is serialized on OJ.

/**
 * 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) {
        // Adopt levelorder traversal
        List<List<Integer>> result = new LinkedList<>();
        
        if (root == null)
            return result;

        List<List<Integer>> tmp = new LinkedList<>();
        List<Integer> row = new LinkedList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        int count = 0;
        int curMode = 1; // Number of nodes of current level
        int nxtMode = 0; // Number of nodes of next level       
        
        queue.add(root);
        
        while (!queue.isEmpty())
        {
            TreeNode node = queue.remove();
            
            row.add(node.val);
            count++;
            
            // Verify if current node is the last node of the level or not
            if (count%curMode == 0)
            {
                tmp.add(row);
                row = new LinkedList<>();
                count = 0;
            }
            
            if (node.left != null)
            {
                queue.add(node.left);
                nxtMode++; // Count the number of next level node
            }
                
            if (node.right != null)
            {
                queue.add(node.right);
                nxtMode++;
            }
            
            // When reached the end of the level, assign next mode to current mode.
            // Next mode then reset to 0.            
            if (count == 0)
            {
                curMode = nxtMode;
                nxtMode = 0;
            }
        }
        
        // Reverse the order
        for (int i = tmp.size()-1; i >= 0; i--)
            result.add(tmp.get(i));
        
        return result;
    }
}