[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],
]

[Solution].java

/**
 * 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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        LinkedList<ArrayList<Integer>> all = new LinkedList<ArrayList<Integer>>();
        if(root == null) return new ArrayList<ArrayList<Integer>>(all);
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        while(!q.isEmpty()){
            ArrayList<Integer> one = new ArrayList<Integer>();
            for(TreeNode node: q) one.add(node.val);
            all.addFirst(one);

            Queue<TreeNode> qNext = new LinkedList<TreeNode>();
            while(!q.isEmpty()){
                TreeNode node = q.poll();
                if(node.left != null) qNext.offer(node.left);
                if(node.right != null) qNext.offer(node.right);
            }
            q = qNext;
        }
        return new ArrayList<ArrayList<Integer>>(all);
    }
}