不带头结点的单链表面试汇总

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import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.HashMap;

import org.apache.commons.io.IOUtils;

//不带头结点的链表
public class LinkedList3
{
    @SuppressWarnings("unchecked")
    public static void main(String[] args) throws Exception
    {
        LinkedList3 list = new LinkedList3(1, 2, 3, 4, 5);
        // System.out.println(list.isEmpty());
        // System.out.println(list.size());
        System.out.println(list);

        // Entry entry = list.getEntry(2);

        // LinkedList3 n_list = (LinkedList3) list.clone();
        // System.out.println(n_list);
        // LinkedList3.reverseListByIterator(n_list);
        // LinkedList3.reverseListByRecursion(n_list);
        // System.out.println(n_list);

        // Entry entry = LinkedList3.lastIndexOf(list, 6);
        // Entry entry = LinkedList3.lastIndexOfByArray(list, 2);
        // Entry entry = LinkedList3.getMiddleEntry(list);

        // LinkedList3.reversePrintListByStack(list);
        // LinkedList3.reversePrintListByRecursion(list);

        // LinkedList3 list1 = new LinkedList3();
        // LinkedList3 list2 = new LinkedList3(2, 3, 4, 5);
        // System.out.println(list1);
        // System.out.println(list2);
        // LinkedList3 list3 = LinkedList3.mergeList(list1, list2);
        // System.out.println(list3);

        // LinkedList3 list1 = new LinkedList3(1, 2, 3, 4);
        // list1.getEntry(3).setNext(list1.getEntry(3));
        // System.out.println(LinkedList3.hasLoop(list1));
        // Entry entry = LinkedList3.getFirstEnterLoopEntry(list1);
        // Entry entry = LinkedList3.getFirstEnterLoopEntry2(list1);

        // LinkedList3 list1 = new LinkedList3(1, 2, 3, 4, 5);
        // LinkedList3 list2 = new LinkedList3(11, 22, 33, 44,
        // 55);
        // list2.getEntry(1).setNext(list1.getEntry(0));
        // list2.getEntry(3).setNext(list1.getEntry(2));
        // System.out.println(list1);
        // System.out.println(list2);
        // System.out.println(LinkedList3.isIntersect(list1, list2));
        // System.out.println(LinkedList3.isIntersect2(list1, list2));
        // System.out.println(LinkedList3.isIntersect3(list1, list2));
        //
        // Entry entry = LinkedList3.getFirstIntersectEntry(list1,
        // list2);
        // System.out.println(entry == null ? "not found" : entry.element);
        // LinkedList3.deleteEntry(list, list.getEntry(0));
        // System.out.println(list);
    }

    // 头指针
    private Entry head;

    public LinkedList3()
    {
        //初始化头指针为空
        head = null;
    }

    public LinkedList3(T... elements)
    {
        this();
        Entry tail = head;
        for (T element : elements)
        {
            Entry newEntry = new Entry(element);
            //第一次头指针为空
            if (head == null)
            {
                head = newEntry;
                tail = head;
            }
            else
            {
                //头指针不为空，直接用尾指针来添加节点
                tail.next = newEntry;
                tail = newEntry;
            }
        }
    }

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

    // 1. 求单链表中结点的个数
    public int size()
    {
        if (isEmpty())
            return 0;

        Entry curEntry = head;
        int c = 0;
        while (curEntry != null)
        {
            c++;
            curEntry = curEntry.next;
        }
        return c;
    }

    // 2. 将单链表反转: reverseList（遍历）
    public static  void reverseListByIterator(LinkedList3 list)
    {
        Entry curEntry = list.head;
        Entry nextEntry = null;
        Entry nextnextEntry = null;

        if (curEntry == null || curEntry.next == null)
            return;

        while (curEntry.next != null)
        {
            nextEntry = curEntry.next;
            nextnextEntry = nextEntry.next;
            nextEntry.next = list.head;
            list.head = nextEntry;
            curEntry.next = nextnextEntry;
        }
    }

    // 2. 将单链表反转: reverseListRec（递归）
    public static  void reverseListByRecursion(LinkedList3 list)
    {
        if (list.head == null || list.head.next == null)
            return;
        list.head = reverseList(list.head);
    }

    private static  Entry reverseList(Entry entry)
    {
        if (entry.next == null)
            return entry;

        Entry newHead = reverseList(entry.next);
        entry.next.next = entry;
        entry.next = null;
        return newHead;
    }

    // 3. 查找单链表中的倒数第K个结点（k > 0）
    public static  Entry lastIndexOf(LinkedList3 list, int n)
    {
        if (n <= 0)
            throw new IllegalArgumentException("The input number should be greater than zero");

        Entry first = list.getEntry(n - 1);
        Entry second = list.head;

        while (first.next != null)
        {
            first = first.next;
            second = second.next;
        }
        return second;
    }

    // 3. 查找单链表中的倒数第K个结点（k > 0）,数组实现
    @SuppressWarnings("unchecked")
    public static  Entry lastIndexOfByArray(LinkedList3 list, int n)
    {
        if (n <= 0)
            throw new IllegalArgumentException("The input number should be greater than zero");

        Object[] storage = new Object[n];
        int index = 0, c = 0;
        Entry curEntry = list.head;

        while (curEntry != null)
        {
            storage[index] = curEntry;
            curEntry = curEntry.next;
            index = (index + 1) % n;
            c++;
        }

        if (storage[index] == null)
            throw new IllegalStateException("less than " + n + " elemements, current is " + c);
        return Entry.class.cast(storage[index]);
    }

    // 4. 查找单链表的中间结点
    public static  Entry getMiddleEntry(LinkedList3 list)
    {
        if (list.isEmpty())
            return null;

        Entry first = list.head;
        Entry second = list.head;

        while (first.next != null && first.next.next != null)
        {
            first = first.next.next;
            second = second.next;
        }
        return second;
    }

    // 5. 从尾到头打印单链表,通过栈实现
    public static  void reversePrintListByStack(LinkedList3 list)
    {
        java.util.LinkedList> stack = new java.util.LinkedList>();
        Entry curEntry = list.head;

        while (curEntry != null)
        {
            stack.push(curEntry);
            curEntry = curEntry.next;
        }

        String result = "[";
        while (!stack.isEmpty())
            result += stack.pop().element + ", ";

        if (result.length() > 1)
            result = result.substring(0, result.length() - 2);
        result += "]";
        System.out.println(result);
    }

    // 5. 从尾到头打印单链表（递归）
    public static  void reversePrintListByRecursion(LinkedList3 list)
    {
        String result = reversePrintList(list.head);
        if (result.length() > 0)
            result = result.substring(0, result.length() - 2);
        result = "[" + result + "]";
        System.out.println(result);
    }

    private static  String reversePrintList(Entry entry)
    {
        if (entry == null)
            return "";
        String result = reversePrintList(entry.next);
        result += entry.element;
        return result + ", ";
    }

    // 6. 已知两个单链表list1和list2各自有序，把它们合并成一个链表依然有序
    @SuppressWarnings("unchecked")
    public static  LinkedList3 mergeList(LinkedList3 list1, LinkedList3 list2)
    {
        Entry curEntry1 = list1.head;
        Entry curEntry2 = list2.head;
        Entry newHead = null;
        Entry curEntry = newHead;

        //先处理新的头指针为空的情况
        if (curEntry1 != null && curEntry2 != null)
        {
            if (((Comparable) curEntry1.element).compareTo(curEntry2.element) < 0)
            {
                newHead = new Entry(curEntry1.element, null);
                curEntry1 = curEntry1.next;
            }
            else
            {
                newHead = new Entry(curEntry2.element, null);
                curEntry2 = curEntry2.next;
            }
            curEntry = newHead;
        }
        else if (curEntry1 != null)
        {
            newHead = new Entry(curEntry1.element, null);
            curEntry = newHead;
            curEntry1 = curEntry1.next;
        }
        else if (curEntry2 != null)
        {
            newHead = new Entry(curEntry2.element, null);
            curEntry = newHead;
            curEntry2 = curEntry2.next;
        }

        while (curEntry1 != null && curEntry2 != null)
        {
            if (((Comparable) curEntry1.element).compareTo(curEntry2.element) < 0)
            {
                curEntry.next = new Entry(curEntry1.element, null);
                curEntry = curEntry.next;
                curEntry1 = curEntry1.next;
            }
            else
            {
                curEntry.next = new Entry(curEntry2.element, null);
                curEntry = curEntry.next;
                curEntry2 = curEntry2.next;
            }
        }

        while (curEntry1 != null)
        {
            curEntry.next = new Entry(curEntry1.element, null);
            curEntry = curEntry.next;
            curEntry1 = curEntry1.next;
        }

        while (curEntry2 != null)
        {
            curEntry.next = new Entry(curEntry2.element, null);
            curEntry = curEntry.next;
            curEntry2 = curEntry2.next;
        }

        LinkedList3 result = new LinkedList3();
        result.setHead(newHead);
        return result;
    }

    // 7. 判断一个单链表中是否有环
    public static  boolean hasLoop(LinkedList3 list)
    {
        Entry first = list.head;
        Entry second = list.head;

        while (first != null && first.next != null)
        {
            first = first.next.next;
            second = second.next;
            if (first == second)
                return true;
        }
        return false;
    }

    // 8. 已知一个单链表中存在环，求进入环中的第一个节点
    public static  Entry getFirstEnterLoopEntry(LinkedList3 list)
    {
        Entry first = list.head;
        Entry second = list.head;

        while (first != null && first.next != null)
        {
            first = first.next.next;
            second = second.next;
            if (first == second)
                break;
        }

        if (first == null || first.next == null)
            return null;

        second = list.head;
        while (first != second)
        {
            first = first.next;
            second = second.next;
        }
        return first;
    }

    // 8. 已知一个单链表中存在环，求进入环中的第一个节点 HashMap实现
    public static  Entry getFirstEnterLoopEntry2(LinkedList3 list)
    {
        HashMap, Boolean> storage = new HashMap, Boolean>();
        Entry curEntry = list.head;

        while (curEntry != null)
        {
            if (storage.get(curEntry) == Boolean.TRUE)
                return curEntry;
            else
            {
                storage.put(curEntry, Boolean.TRUE);
                curEntry = curEntry.next;
            }
        }
        return null;
    }

    // 9. 判断两个单链表是否相交 HashMap实现
    public static  boolean isIntersect(LinkedList3 list1, LinkedList3 list2)
    {
        if (list1.isEmpty() || list2.isEmpty())
            return false;

        HashMap, Boolean> storage = new HashMap, Boolean>();
        Entry curEntry1 = list1.head;
        while (curEntry1 != null)
        {
            storage.put(curEntry1, Boolean.TRUE);
            curEntry1 = curEntry1.next;
        }

        Entry curEntry2 = list2.head;
        while (curEntry2 != null)
        {
            if (storage.get(curEntry2) == Boolean.TRUE)
                return true;
            curEntry2 = curEntry2.next;
        }
        return false;
    }

    // 9. 判断两个单链表是否相交 如果相交 让第一个链表的next指向第二个链表的头指针，可以构成一个环
    public static  boolean isIntersect2(LinkedList3 list1, LinkedList3 list2)
    {
        if (list1.isEmpty() || list2.isEmpty())
            return false;

        Entry curEntry1 = list1.head;
        Entry curEntry2 = list2.head;

        while (curEntry1.next != null)
            curEntry1 = curEntry1.next;

        curEntry1.next = list2.head;
        boolean exist = false;
        while (curEntry2.next != null)
        {
            if (curEntry2.next == list2.head)
            {
                exist = true;
                break;
            }
            curEntry2 = curEntry2.next;
        }
        curEntry1.next = null;
        return exist;
    }

    // 9. 判断两个单链表是否相交 如果相交 最后一个节点一定相等
    public static  boolean isIntersect3(LinkedList3 list1, LinkedList3 list2)
    {
        if (list1.isEmpty() || list2.isEmpty())
            return false;

        Entry curEntry1 = list1.head;
        Entry curEntry2 = list2.head;

        while (curEntry1.next != null)
            curEntry1 = curEntry1.next;

        while (curEntry2.next != null)
            curEntry2 = curEntry2.next;

        if (curEntry1 == curEntry2)
            return true;
        return false;
    }

    // 10. 求两个单链表相交的第一个节点
    public static  Entry getFirstIntersectEntry(LinkedList3 list1, LinkedList3 list2)
    {
        int m = list1.size();
        int n = list2.size();

        Entry curEntry1 = list1.head;
        Entry curEntry2 = list2.head;
        int c;
        if (m > n)
        {
            c = m - n;
            while (c != 0)
            {
                curEntry1 = curEntry1.next;
                c--;
            }
        }
        else
        {
            c = n - m;
            while (c != 0)
            {
                curEntry2 = curEntry2.next;
                c--;
            }
        }

        while (curEntry1 != null)
        {
            if (curEntry1 == curEntry2)
                return curEntry1;
            curEntry1 = curEntry1.next;
            curEntry2 = curEntry2.next;
        }
        return null;
    }

    // 11. 给出一单链表头指针pHead和一节点指针pToBeDeleted，O(1)时间复杂度删除节点pToBeDeleted: delete
    public static  void deleteEntry(LinkedList3 list, Entry entry)
    {
        if (entry == null)
            return;

        if (entry.next != null)
        {
            entry.element = entry.next.element;
            entry.next = entry.next.next;
        }
        else
        {
            if (entry == list.head)
                list.head = entry.next;
            else
            {
                Entry preEntry = list.head;
                while (preEntry != null)
                {
                    if (preEntry.next == entry)
                        break;
                    preEntry = preEntry.next;
                }
                preEntry.next = preEntry.next.next;
            }
        }
    }

    public void setHead(Entry head)
    {
        this.head = head;
    }

    public Entry createEntry(T element)
    {
        return new Entry(element);
    }

    public Entry getEntry(int index)
    {
        if (index < 0)
            throw new IllegalArgumentException("index should be a positive number or zero");

        int c = 0;
        Entry curEntry = head;
        while (curEntry != null && c < index)
        {
            c++;
            curEntry = curEntry.next;
        }

        if (curEntry == null)
            throw new IllegalStateException("less than " + (index + 1) + " elements, current is " + c);
        return curEntry;
    }

    @SuppressWarnings("unchecked")
    @Override
    public Object clone() throws CloneNotSupportedException
    {
        try
        {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            ObjectOutputStream oos = new ObjectOutputStream(baos);
            try
            {
                oos.writeObject(head);
                oos.flush();

                ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(baos.toByteArray()));
                try
                {
                    LinkedList3 result = new LinkedList3();
                    result.setHead(Entry.class.cast(ois.readObject()));
                    return result;
                }
                finally
                {
                    IOUtils.closeQuietly(ois);
                }
            }
            finally
            {
                IOUtils.closeQuietly(oos);
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
            return super.clone();
        }
    }

    @Override
    public String toString()
    {
        String result = "[";
        Entry curEntry = head;
        while (curEntry != null)
        {
            result += curEntry.element;
            curEntry = curEntry.next;
            if (curEntry != null)
                result += ", ";
        }
        return result + "]";
    }

    public static class Entry implements Serializable
    {
        private static final long serialVersionUID = 1L;
        T element;
        Entry next;

        Entry(T element)
        {
            this(element, null);
        }

        Entry(T element, Entry next)
        {
            this.element = element;
            this.next = next;
        }

        public void setElement(T element)
        {
            this.element = element;
        }

        public T getElement()
        {
            return element;
        }

        public void setNext(Entry next)
        {
            this.next = next;
        }

        public Entry getNext()
        {
            return next;
        }

        @Override
        public String toString()
        {
            return String.valueOf(element);
        }
    }
}