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

 

public class Solution {
    public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
		if(root != null){
			Queue<TreeNode> topLevel = new LinkedList<TreeNode>();
			topLevel.add(root);
			while(topLevel.peek() != null){
				Queue<TreeNode> nextLevel = new LinkedList<TreeNode>();
				List<Integer> value = new ArrayList<Integer>();
				while(topLevel.peek() != null){
					TreeNode node = topLevel.poll();
					value.add(node.val);
					if(node.left != null)
						nextLevel.add(node.left);
					if(node.right != null)
						nextLevel.add(node.right);
				}
				result.add(0, value);
				topLevel = nextLevel;
			}
		}
		return result;    
    }
}