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package util;import java.util.NoSuchElementException;/** * @Author lyr * @create 2020/6/28 13:13 */public class LinkedList {
       static class Node {
           private int value;        private Node next;        public Node(int value) {
               this.value = value;        }    }    private Node first;    private Node last;    /**     * addFirst     * addLast     * deleteFirst     * deleteLast     * contains     * indexOf     */    public void addLast(int item) {
           Node n = new Node(item);        if (isEmpty()) {
               first = last = n;        } else {
               last.next = n;            last = n;        }    }    public void addFirst(int item) {
           Node node = new Node(item);        if (isEmpty()) {
               first = last = node;        } else {
               node.next = first;            first = node;        }    }    public int indexOf(int item) {
           int index = 0;        Node current = first;        while (current != null) {
               if (current.value == item) return index;            index++;            current = current.next;        }        return -1;    }    public boolean contains(int item) {
           return indexOf(item) != -1;    }    public void removeFirst() {
           if (isEmpty()) {
               throw new NoSuchElementException();        }        if (first == last) {
               first = last = null;        }        // [10,20,30]        //first ->20        Node second = first.next;        //移除连接        first.next = null;        first = second;    }    /**     * remove the last element     */    public void removeLast() {
           if (isEmpty()) {
               throw new NoSuchElementException();        }        if (first == last) {
               first = last = null;            return;        }        Node previous = getPrevious(last);        last = previous;        last.next = null;    }    private Node getPrevious(Node node) {
           Node current = first;        while (current != null) {
               if (current.next == node) return current;            current = current.next;        }        return null;    }    private boolean isEmpty() {
           return this.first == null;    }}

后续补充

public int getKthFromTheEnd(int k) {
           if(isEmpty()) throw new IllegalStateException();        var a  = first;        var b = a;        for(var i = 0;i
   
     ");            it = it.next;        }        return sb.append("null").toString();    }
   

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