如何为二叉搜索树实现我的 BSTIterator 类？

Question

所以在我的二叉搜索树中我做了这个类（但我不确定是否所有的方法都正确）：

import java.util.Iterator;

public class BSTRefBased extends AbstractBinaryTree 
    implements Iterable<WordRefs>
{
    private TreeNode root;


    public BSTRefBased() {
    }


    public BSTRefBased(WordRefs item,
        AbstractBinaryTree left,
        AbstractBinaryTree right)
    {
        root = new TreeNode(item, null, null);
        if (left != null) {
            attachLeftSubtree(left);
        }

        if (right != null) {
            attachRightSubtree(right);
        }
    }


    public boolean isEmpty() {
        return (root == null);
    }


    public void makeEmpty() {
        root = null;
    }


    protected TreeNode getRoot() {
        return root;
    }


    protected void setRoot(TreeNode r) {
        this.root = r;
    }


    public WordRefs getRootItem() throws TreeException {
        if (root == null) {
            throw new TreeException("getRootItem() on empty tree");
        }

        return root.item;
    }


    public void setRootItem(WordRefs item) {
        if (root == null) {
            root = new TreeNode(item);
        } else {
            root.item = item;
        }
    }


    public void attachLeft(WordRefs item) throws TreeException {
        if (isEmpty()) {
            throw new TreeException("attachLeft to empty tree");
        }

        if (!isEmpty() && root.left != null) {
            throw new TreeException("attachLeft to occupied left child");
        }

        root.left = new TreeNode(item, null, null);

        return;
    } 


    public void attachLeftSubtree(AbstractBinaryTree left) {
        if (isEmpty()) {
            throw new TreeException("attachLeftSubtree to empty tree");
        }

        if (!isEmpty() && root.left != null) {
            throw new 
                TreeException("attachLeftSubtree to occupied right child");
        }

        root.left = left.getRoot();
        left.makeEmpty();

        return;    
    }


    public void attachRight(WordRefs item) throws TreeException {
        if (isEmpty()) {
            throw new TreeException("attachRight to empty tree");
        }

        if (!isEmpty() && root.right != null) {
            throw new TreeException("attachRight to occupied right child");
        }

        root.right = new TreeNode(item, null, null);

        return;
    } 


    public void attachRightSubtree(AbstractBinaryTree right) {
        if (isEmpty()) {
            throw new TreeException("attachRightSubtree to empty tree");
        }

        if (!isEmpty() && root.right != null) {
            throw new 
                TreeException("attachRightSubtree to occupied right child");
        }

        root.right = right.getRoot();
        right.makeEmpty();

        return;
    }


    public AbstractBinaryTree detachLeftSubtree()
        throws TreeException 
    {
        if (root == null) {
            throw new TreeException("detachLeftSubtree on empty tree");
        }

        BSTRefBased result = new BSTRefBased();
        result.setRoot(root.left);
        root.left = null;

        return result;
    }


    public AbstractBinaryTree detachRightSubtree()
        throws TreeException
    {
        if (root == null) {
            throw new TreeException("detachLeftSubtree on empty tree");
        }

        BSTRefBased result = new BSTRefBased();
        result.setRoot(root.right);
        root.right = null;

        return result;
    }


    public void insert(String word) {
        root=insertItem(root, word);
    }


    protected TreeNode insertItem(TreeNode r, String word) {
        if(r==null){
            r = new TreeNode(new WordRefs(word));
            r.left = null;
            r.right = null;
        }
        else if(word.compareTo(r.item.getWord()) < 0){
            r.left = insertItem(r.left, word);
        } else if (word.compareTo(r.item.getWord()) > 0){
            r.left = insertItem(r.right, word);
        }
        return r;
    }


    public WordRefs retrieve(String word) {
        TreeNode node = retrieveItem(root, word);
        if (node == null) {
            return null;
        } else {
            return new WordRefs(node.item.getWord());
        }
    }


    protected TreeNode retrieveItem(TreeNode r, String word) {
        if (r == null){
            return null;
        }
        if (word.compareTo(r.item.getWord()) < 0){
            return retrieveItem(r.left, word);
        } else if (word.compareTo(r.item.getWord()) > 0 ){
            return retrieveItem(r.right, word);
        }
        return r;
    }


    public void delete(String word) {
        root = deleteItem(root, word);
    }


    protected TreeNode deleteItem(TreeNode r, String word) {
        if (r == null){
            return r;
        }
        if(word.compareTo(r.item.getWord()) < 0){
            r.left = deleteItem(r.left, word);
        } else if (word.compareTo(r.item.getWord()) > 0){
            r.right = deleteItem(r.right, word);
        } else if(r.left != null && r.right != null)
            {
                root = findLeftMost(r.right);
                r.right = deleteItem(r.right, word).right;
            } else {
                return r;
            }
        return r;
    }


    protected TreeNode deleteNode(TreeNode node) {
        node = null;
        return node;
    }


    protected TreeNode findLeftMost(TreeNode node) {
        if(node == null){
            return null;
        } else {
            return findLeftMost(node.left);
        }
    }


    protected TreeNode deleteLeftMost(TreeNode node) {
        if (node == null){
            return null;
        } else if(node.left == null){
            return findLeftMost(node.left);
        } else {
            return deleteNode(node.left);
        }
    }


    public Iterator<WordRefs> iterator() {
        return new BSTIterator(this);
    }


    public static void main(String args[]) {
        BSTRefBased t;
        AbstractBinaryTree tt;
        int i;
        boolean result;
        String message;

        message = "Test 1: inserting 'humpty' -- ";
        t = new BSTRefBased();
        try {
            t.insert("humpty");
            result = t.getRootItem().getWord().equals("humpty");
        } catch (Exception e) {
            result = false;
        }
        System.out.println(message + (result ? "passed" : "FAILED"));

        message = "Test 2: inserting 'humpty', 'dumpty', 'sat' -- ";
        t = new BSTRefBased();
        try {
            t.insert("humpty");
            t.insert("dumpty");
            t.insert("sat");
            result = t.getRootItem().getWord().equals("humpty");
            tt = t.detachLeftSubtree();
            result &= tt.getRootItem().getWord().equals("dumpty");
            tt = t.detachRightSubtree();
            result &= tt.getRootItem().getWord().equals("sat");
        } catch (Exception e) {
            result = false;
        }
        System.out.println(message + (result ? "passed" : "FAILED"));

        message = "Test 3: deleting 'humpty' -- ";
        t = new BSTRefBased();
        try {
            t.delete("humpty");
            result = t.getRootItem().getWord().equals(null);
        } catch (Exception e) {
            result = false;
        }
        System.out.println(message + (result ? "passed" : "FAILED"));

                message = "Test 4: deleting 'dumpty' 'sat' -- ";
        t = new BSTRefBased();
        try {
            t.delete("dumpty");
            t.delete("sat");
            result = t.getRootItem().getWord().equals(null);
            tt = t.detachLeftSubtree();
            result &= tt.getRootItem().getWord().equals(null);
        } catch (Exception e) {
            result = false;
        }
        System.out.println(message + (result ? "passed" : "FAILED"));
    }
} 

所以我要做的是实现另一个名为 BSTIterator 的类，它看起来像这样（请注意，底部有 3 个方法填写但它们可能不正确）

import java.util.LinkedList;

public class BSTIterator implements java.util.Iterator<WordRefs> {
    private BSTRefBased t;
    private WordRefs currentItem;
    private LinkedList<WordRefs> list;

    public BSTIterator(BSTRefBased t) {
        this.t = t;
        currentItem = null;
        list = new LinkedList<>();
        setInorder();
    }

    public boolean hasNext() {
        return false;
    }

    public WordRefs next() throws java.util.NoSuchElementException {
        return null;
    }

    public void remove() throws UnsupportedOperationException {
        throw new UnsupportedOperationException();
    }

    public void setPreorder() {
    }

    public void setInorder() {
    }

    public void setPostorder() {
    }

    private void preorder(TreeNode node) {
        if(node == null)
        {       
            return;
        }
        preorder(node.left);
        preorder(node);
        preorder(node.right);
    }

    private void inorder(TreeNode node) {
        if(node == null)
        {       
            return;
        }
        inorder(node.left);
        inorder(node);
        inorder(node.right);
    }

    private void postorder(TreeNode node) {
        if(node == null)
        {       
            return;
        }
        postorder(node.left);
        postorder(node);
        postorder(node.right);
    }
}

我只是不确定如何开始实现该类中的特定方法，例如 setpreorder。那么这些方法背后的基本思想是什么？

Answer 1

顺序是遍历二叉树的方式

inorder == left node, the actual node, right node
preorder == the actual node, left node,right node
postorder == left node, right node, the actual node

更好的图片示例

例如 setInOrder 方法用于在你的类中设置你的链表以接受数据的遍历顺序，或者重置链表

public void setInorder() {
      list = new LinkedList<>();
    }

然后你会像这样调用 inorder 方法，但将数据递归地添加到列表中左、节点、右

 private void inorder(TreeNode node) {
        if(node != null)
        {       
        inorder(node.left); //traverse left first
        list.add(node.item); //traverse the actual nodes data
        inorder(node.right); // then traverse right
        }

    }

希望对你有帮助，剩下的留给你解决