犯了两个错误，一个是node为null时，仍然把left和right往queue里放，空指针错误。一个是到最后一层，节点的left和right都是null，那么新建的ArrayList就是空，不该往结果里放。

此题DFS也可，就是把层次传进去：

public class Solution {    public ArrayList
     
      
       > levelOrderBottom(TreeNode root) {        ArrayList
       
        
         > ans = new ArrayList
         
          
           >(); Queue
           
             queue = new LinkedList
            
             (); queue.offer(root); int lastLevel = 1; int thisLevel = 0; ArrayList
             
               tmp = new ArrayList
              
               (); while (queue.size() != 0) { TreeNode n = queue.poll(); lastLevel--; if (n != null) { tmp.add(n.val); queue.offer(n.left); queue.offer(n.right); thisLevel += 2; } if (lastLevel == 0) { lastLevel = thisLevel; thisLevel = 0; if (tmp.size() != 0) { ans.add(tmp); } tmp = new ArrayList
               
                (); } } int len = ans.size(); for (int i = 0; i < len/2; i++) { tmp = ans.get(i); ans.set(i, ans.get(len - i - 1)); ans.set(len - i - 1, tmp); } return ans; }}