链表删除和insertAfter方法

Question

我在使用 delete 和 insertAfter 方法时遇到问题，这些方法是执行操作并在操作成功（真）或不成功（假）时返回布尔值。 InsertAfter 方法会插入一个字符串但总是在同一个位置，Delete 方法总是会删除同一个节点。

private class StrNode {

        String data;
        StrNode next;
    }

    private StrNode head;   // the head of the singly-linked list.

    public StringList() {
        head = null;
    }

public void prepend(String s) {                                                         
        var newNode = new StrNode();
        // TODO: Adds an item to the start of the list.     
        newNode.data = s;
        if(head == null) {
            head = newNode;
        }
        else {
        newNode.next = head;
        head = newNode;
        }
        
    }

/**
     * Inserts an item after the first instance of a key if the key exists.
     *
     * @param s the item to insert
     * @param key the item in the list to insert after
     * @return whether the insertion was successful
     */
    public boolean insertAfter(String s, String key) {                                                      
        // TODO:    Inserts an item after the first instance of a key if the key exists.
        var newNode = new StrNode();
        StrNode current = head;
        newNode.data = s;
        
        if(head == null) {
            head = newNode;
        }
        else if(current == newNode.next){
            current.next = newNode;
            current = newNode;

        }
        else {
            newNode.next = current.next;
            current.next = newNode;
        }
        
        
        return false;
    }
    
    /**
     * Deletes the first instance of an item from the list.
     *
     * @param key the value of the item to delete from the list.
     * @return whether the deletion was successful.
     */
    public boolean delete(String key) {                                                     
        // TODO:    Deletes the first instance of an item from the list.
        StrNode current = head;
        StrNode sucNode = current;
        
        if(current == null) {
            sucNode = head.next;
            head = sucNode;
            return true;
        }
        else if(current.next != null) {
            sucNode = current.next.next;
            current.next = sucNode;
            return true;
        }

        return false;
    }

我想在三之后插入四的主要方法应该是：三、二、四、一。 但我得到：三、四、二、一

delete 方法只是删除实际上应该看起来像这样的四个：三、四、二 但我得到：三，二，一 主要：

public static void main(String[] args) {
        
        StringList s = new StringList();
        
        s.prepend("one");
        s.prepend("two");
        s.prepend("three");
        System.out.println(s);
        
        s.insertAfter("four", "three");
        System.out.println(s);
        
        System.out.println(s.delete("one"));
        System.out.println(s);
        
        
    }

Answer 1

您询问了两种方法：

insertAfter

一些问题：

• 您不使用参数key。
• 如果head 为空，则永远无法满足将节点插入具有键的节点的条件，因此在这种情况下，您应该返回false。
• current 被初始化为 head，因此您将新节点分配给 head.next 而不检查 key 匹配...
• 你总是返回false，从不返回true

您应该通过遍历列表来查找给定的键：

public boolean insertAfter(String s, String key) {
    // Inserts an item after the first instance of a key if the key exists.
    StrNode current = head;
    
    while (current != null) {
        if (current.data == key) { // found the insertion spot
            var newNode = new StrNode();
            newNode.data = s;
            newNode.next = current.next;
            current.next = newNode;
            return true;
        }
        current = current.next; // need to walk along the list
    }
    return false; // didn't find the key
}

delete

• 您没有使用参数key。
• 如果head（即current）为null，则永远无法满足要删除的节点应具有给定键的条件，因此在这种情况下应返回false。
• 在另一种情况下，您总是删除第二个节点而不检查 key 匹配...

更正：

public boolean delete(String key) {                                                     
    // Deletes the first instance of an item from the list.
    StrNode current = head;
    if (head == null) return false;
    if (head.data == key) {
         head = head.next;
         return true;
    }

    while (current.next != null) {
        if (current.next.data == key) { // found it
            current.next = current.next.next; // delete it
            return true;
        }
        current = current.next; // need to walk along the list
    }
    return false; // not found
}

关于prepend的备注：

您不需要if 语句。当head 为空时，else 块中的代码可以正常工作，因此您的代码可以是：

public void prepend(String s) { 
    // Adds an item to the start of the list. 
    StrNode node = new StrNode();
    node.data = s;
    node.next = head;
    head = node;
}