通用BinarySearchTree中的PostOrder輸出（Java）

Question

我在編寫一些作業時遇到了一些麻煩。我應該編寫一個通用二進制搜索樹實用程序，其中包括一個用於返回Tree的postOrder traversion的ArrayList的方法。我的代碼編譯，但它會拋出一個NullPointerException除了空的樹。我的錯誤在哪裏？

public ArrayList<T> postOrder(BinarySearchTree<T> tree) { 
    if (tree == null) { 
     return null; 
    } else { 
     ArrayList<T> post = new ArrayList<T>(); 
     post.addAll(postOrder(tree.left)); 
     post.addAll(postOrder(tree.right)); 
     post.add(tree.thing); 
     return post; 
    } 
} 

類BinarySearchTree是：

public class BinarySearchTree<T> { 
/** 
* The key by which the thing is refered to. Must be unique. 
*/ 
public int key; 

/** 
* The thing itself. 
*/ 
public T thing; 

/** 
* The left sub-tree 
*/ 
public BinarySearchTree<T> left; 

/** 
* The right sub-tree 
*/ 
public BinarySearchTree<T> right; 
Biny 
/** 
* Create a new binary search tree without children. 
* @param key the key by which the thing is refered to 
* @param thing the new thing 
*/ 
public BinarySearchTree(int key, T thing) 
{ 
    this.key = key; 
    this.thing = thing; 
    this.left = null; 
    this.right = null; 
} 

/** 
* Create a new binary search tree 
* @param key the key by which the thing is refered to 
* @param thing the thing which is managed by the new binary search tree 
* @param left the left sub-tree of the new binary search tree 
* @param right the right sub-tree of the new binary search tree 
*/ 
public BinarySearchTree(int key, T thing, BinarySearchTree<T> left, BinarySearchTree<T> right) 
{ 
    this.key = key; 
    this.thing = thing; 
    this.left = left; 
    this.right = right; 
} 

感謝您的幫助

編輯：我測試我的代碼以絃樂，但希望不要因爲使用泛型類型的關係。

Answer 1

試試這個：

public ArrayList<T> postOrder(BinarySearchTree<T> tree) { 
    if (tree == null) { 
     return null; 
    } else { 
     ArrayList<T> post = new ArrayList<T>(); 
     ArrayList<T> l = postOrder(tree.left); 
     if (l != null) post.addAll(l); 
     ArrayList<T> r = postOrder(tree.right); 
     if (r != null) post.addAll(r); 
     post.add(tree.thing); 
     return post; 
    } 
}