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.

Solution:
Use bfs to traverse the tree level by level, and then reverse the results from top-bottom to bottom-up.

Note:

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 *
public class Solution {
    public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root) {
        ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
        ArrayList<TreeNode> queue = new ArrayList<TreeNode>();
        if(root == null) return results;
        queue.add(root);
        int currentLevelCount = 1;   // nodes number in the current level
        int nextLevelCount = 0;     // nodes number in the next level
        while(queue.size() > 0){
            ArrayList<Integer> oneLevel = new ArrayList<Integer>();
            while(currentLevelCount > 0){
                TreeNode currentNode = queue.get(0);
                if(currentNode.left != null){
                    queue.add(currentNode.left);
                    nextLevelCount++;
                }
                if(currentNode.right != null){
                    queue.add(currentNode.right);
                    nextLevelCount++;
                }
                oneLevel.add(currentNode.val);
                queue.remove(0);
                currentLevelCount--;
            }
            currentLevelCount = nextLevelCount;
            nextLevelCount = 0;    // update
            results.add(oneLevel);
        }
        return reverse(results);   // reverse the results
    }
   
    public ArrayList<ArrayList<Integer>> reverse(ArrayList<ArrayList<Integer>> results){
        ArrayList<ArrayList<Integer>> newResults = new ArrayList<ArrayList<Integer>>();
        for(int i = results.size()-1; i >= 0; i--){
            newResults.add(results.get(i));
        }
        return newResults;
    }
   
   
   
}