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]
]

public List> levelOrderBottom(TreeNode root) {
            List> result=new ArrayList>();
   if(root==null)
    return result;
   List preelement=new ArrayList();
   preelement.add(root);
   Stack> stack=new Stack();
   stack.push(preelement);
 //把每层非空的取出来
   while(true){
    List nowelement=new ArrayList();
    boolean isFinish=false;
    for(TreeNode a : preelement){
    if(a.left!=null){
    isFinish=true;
    nowelement.add(a.left);
    }
    if(a.right!=null){
    isFinish=true;
    nowelement.add(a.right);
    }
   

    //全是空了
    }
    if(!isFinish)
    break;
    stack.push(nowelement);
    preelement=nowelement;
   
   }
   while (!stack.empty()) {
    List element=new ArrayList<>();
    List nodes=stack.pop();
    for(TreeNode aNode:nodes)
    element.add(aNode.val);
result.add(element);
}
   return result;
    }