循环控制-链表删除结点

Posted 2021-01-23 pylearn

0.目录

1.删除结点的思路

2.Java代码实现

• 2.1 链表删除结点的实现
• 2.2 头结点怎么办
• 2.3 测试用例

1.删除结点的思路

链表删除结点的意思就是给定一个值，把链表中与给定值相等的结点全部删除：
例如1 → 2 → 3 → 2 → 5 → Null，删除给定值2，变为：1 → 3 → 5 → Null

删除的思路：

用一个previous指针指向3，如果3的下一个元素是2，就让3指向5（相当于跳过了2），而previous依旧指向3。

PS：本篇结点和创建链表的实现同前文 递归控制-创建链表

2.Java代码实现

2.1 链表删除结点的实现

不考虑头结点时的实现：

    public Node deleteIfEquals(Node head, int value) {
        Node prev = head;
        // Loop invariant: list nodes from head up to prev has been processed.
        // (Nodes with values equal to value are deleted.)
        while (prev.getNext() != null) {
            if (prev.getNext().getValue() == value) {
                // delete it
                prev.setNext(prev.getNext().getNext());
            } else {
                prev = prev.getNext();
            }
        }

        return head;
    }

2.2 头结点怎么办

头结点没有previous怎么办？

1. 特殊处理
2. 增加虚拟头结点

此处采用特殊处理头结点（在Node prev = head;前面加上以下代码）：

        if (head == null) {
            return null;
        }

        if (head.getValue() == value) {
            head = head.getNext();
        }

但是程序依然有问题，比如说2 → 2 → 3 → 2 → 5 → Null，这样开头有两个2，执行以上程序后只会删除第一个2。
优化一下：

        if (head == null) {
            return null;
        }

        while (head.getValue() == value) {
            head = head.getNext();
        }

会发现程序依然有问题，比如说2 → 2 → 2 → 2 → 2 → Null，这样全是2，执行以上程序后head就是null了。这样在while循环中程序会执行null.getValue()，自然会报错。
再优化一下：

        if (head == null) {
            return null;
        }

        while (head != null && head.getValue() == value) {
            head = head.getNext();
        }

运行程序会看到依然有问题，比如说2 → 2 → 2 → 2 → 2 → Null，这样全是2，执行以上程序后head就是null了。这样在接下来的程序中会执行null.getNext()，自然会报错。
再优化一下：

        while (head != null && head.getValue() == value) {
            head = head.getNext();
        }

        if (head == null) {
            return null;
        }

最终的删除节点实现：

    public Node deleteIfEquals(Node head, int value) {
        while (head != null && head.getValue() == value) {
            head = head.getNext();
        }

        if (head == null) {
            return null;
        }

        Node prev = head;
        // Loop invariant: list nodes from head up to prev has been processed.
        // (Nodes with values equal to value are deleted.)
        while (prev.getNext() != null) {
            if (prev.getNext().getValue() == value) {
                // delete it
                prev.setNext(prev.getNext().getNext());
            } else {
                prev = prev.getNext();
            }
        }

        return head;
    }

2.3 测试用例

测试程序是否正确运行：

    public static void main(String[] args) {
        LinkedListCreator creator = new LinkedListCreator();
        LinkedListDeletor deletor = new LinkedListDeletor();
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(1, 2, 3, 2, 5)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2, 2, 3, 2, 2)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2, 2, 2, 2, 2)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(1)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(new ArrayList<Integer>()), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2, 3, 4, 5, 2)), 2));
    }

运行结果为

main所在java文件全部代码：

import interview.common.Node;
import interview.recursion.LinkedListCreator;

import java.util.ArrayList;
import java.util.Arrays;

public class LinkedListDeletor {

    public Node deleteIfEquals(Node head, int value) {
        while (head != null && head.getValue() == value) {
            head = head.getNext();
        }

        if (head == null) {
            return null;
        }

        Node prev = head;
        // Loop invariant: list nodes from head up to prev has been processed.
        // (Nodes with values equal to value are deleted.)
        while (prev.getNext() != null) {
            if (prev.getNext().getValue() == value) {
                // delete it
                prev.setNext(prev.getNext().getNext());
            } else {
                prev = prev.getNext();
            }
        }

        return head;
    }

    public static void main(String[] args) {
        LinkedListCreator creator = new LinkedListCreator();
        LinkedListDeletor deletor = new LinkedListDeletor();
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(1, 2, 3, 2, 5)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2, 2, 3, 2, 2)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2, 2, 2, 2, 2)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(1)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2)), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(new ArrayList<Integer>()), 2));
        Node.printLinkedList(deletor.deleteIfEquals(
            creator.createLinkedList(Arrays.asList(2, 3, 4, 5, 2)), 2));
    }
}