toString() 方法和递归

Question

我正在创建一个使用二叉搜索树实现通用Set 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 方法输出这个二叉树：

(10 (5 (4 () ())   (6 () ()))   (20 (15 () ())   (25 () ())))

() 表示一棵空树。

这是我使用 toString 方法的类。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);
    }

  }

}

Answer 1

我在尝试运行代码时遇到了一些问题，但假设所有问题都已解决 - 以下对我有用：

private String subtreeToString(BTNode r) {
    String str = "";
    if(r == null) {
        return str;
    }
    str += r.item;
    str += " (" + subtreeToString(r.left) + ") (" + subtreeToString(r.right) + ")";
    return str;
}

输出：

10 (5 (4 () ()) (6 () ())) (20 () (25 () ()))

（上面提供的示例输出中还有一个错误 - 应在 6 的子级之后添加右括号）