我正在创建一个Set使用二叉搜索树实现泛型的类。我正在实现我的toString()方法，以便它显示一个括号表达式，但我无法让它工作。例如，这是一个基本的二叉树：
我希望我的 toString 方法这样输出这个二叉树：
(10 (5 (4 () ()) (6 () ())) (20 (15 () ()) (25 () ())))
where()表示一棵空树。
这是我使用 toString 方法的课程。任何帮助表示赞赏！
// Allow short name access to following classes
import csc143.data_structures.*;
public class MySet<E> implements SimpleSet<E> {
// the root of the "tree" that structures the set
private BTNode root;
// the current number of elements in the set
private int numElems;
public MySet() {
root = null;
numElems = 0;
}
/**
* Add an element to the set.
*
* @param e The element to be added to the set.
* @return <tt>true</tt> If this operation updated the contents of the set.
*/
public boolean add(E e) {
try {
root = addToSubtree(root, (Comparable) e);
return true;
} catch(DuplicateAdded exc) {
// duplicate trying to be added
return false;
}
}
// This helper method adds the element "e" to tree rooted at r. Returns
// (possibly new) tree containing "e", or throws DuplicateAdded exception
// if "e" already exists in tree.
private BTNode addToSubtree(BTNode r, Comparable elem)
throws DuplicateAdded {
if(r == null) {
numElems++;
return new BTNode(elem);
}
int compare = elem.compareTo(r.item);
// element already in tree
if(compare == 0) {
throw new DuplicateAdded();
} else if(compare < 0) {
r.left = addToSubtree(r.left, elem);
} else { // compare > 0
r.right = addToSubtree(r.right, elem);
}
// element has been added
return r;
}
/**
* Remove all elements from this set.
*/
public void clear() {
root = null;
numElems = 0;
}
/**
* Checks for the existance of the specified value within the set.
*
* @param e The value sought.
* @return <tt>true</tt> If the value exists in the set.
*/
public boolean contains(E e) {
return subtreeContains(root, (Comparable) e);
}
// This helper method returns whether element "elem" is in
// (sub-)tree with root "r".
private boolean subtreeContains(BTNode r, Comparable elem) {
if(r == null) {
return false;
} else {
int compare = elem.compareTo(r.item);
// found element
if(compare == 0){
return true;
} else if(compare < 0) {
return subtreeContains(r.left, elem);
} else { // compare > 0
return subtreeContains(r.right, elem);
}
}
}
/**
* Check for the existance of elements in the set.
*
* @return <tt>true</tt> If there are no elements in the set.
*/
public boolean isEmpty() {
return root == null;
}
/**
* Return the number of elements in the set.
*
* @return The number of elements in the set.
*/
public int size() {
return numElems;
}
/**
* Returns a String representation of the contents of the set.
*
* @return The String representation of the set.
*/
public String toString() {
return subtreeToString(root);
}
private String subtreeToString(BTNode r) {
String str = "";
if(r == null) {
str += "() ";
return str;
}
str += "(" + r.item;
str += subtreeToString(r.left) +
subtreeToString(r.right) + ")";
return str;
}
// this inner class creates the node that compose the binary tree structure
class BTNode<E> {
/**
* The item stored in the node.
*/
public E item;
/**
* The node to the left of "this" node.
*/
public BTNode left;
/**
* The node to the right of "this" node.
*/
public BTNode right;
/**
* Constructs the BTNode object (three parameters).
*
* @param item The item to be stored in the node.
* @param left The node to the left of "this" node.
* @param right The node to the right of "this" node.
*/
@SuppressWarnings("unchecked")
public BTNode(Object item, BTNode left, BTNode right) {
// bind to references
this.item = (E) item;
this.left = left;
this.right = right;
}
/**
* Constructs the BTNode (one parameter).
*
* @param The item to be stored in the node.
*/
public BTNode(Object item) {
// call three parameter constructor
this(item, null, null);
}
}
}