P107、面试题15：链表中倒数第K个结点

题目：输入一个链表，输出该链表中倒数第K个结点。为了符合大多数人的习惯，本体从1开始奇数，即链表的尾结点是倒数第1个结点。例如一个链表有6个结点，从头结点开始他们的值一次是1、2、3、4、5、6.这个链表的倒数第3个结点是值为4的结点。

思路：先是一个指针往前走，走了k步之后，前后指针一起走，但是要注意特殊情景。

 

测试用例：

1）功能测试（第k个结点在链表的中间，第k个结点是链表的头结点，第k个结点是链表的尾结点）；

2）特殊输入测试（链表头结点为null指针，链表的结点总数少于k，k等于0）。

 

代码实现：

package com.yyq;

/**
 * Created by Administrator on 2015/9/13.
 */
public class FindKthToTail {
    public static ListNode findKthToTail(ListNode pListHead, int k){
        if(pListHead == null || k <= 0)
            return null;
        ListNode pAead = pListHead;
        ListNode pBehind = null;
        for(int i = 0; i < k-1; i++){
            if (pAead.getM_pNext() != null)
                pAead = pAead.getM_pNext();
            else
                return null;
        }
        pBehind = pListHead;
        while(pAead.getM_pNext() != null){
            pAead = pAead.getM_pNext();
            pBehind = pBehind.getM_pNext();
        }
        return pBehind;
    }

    public static void printList(ListNode pListHead){
        if (pListHead == null)
            return;
        ListNode pNode = pListHead;
        while(pNode!= null){
            System.out.print(pNode.getM_nValue()+"  ");
            pNode = pNode.getM_pNext();
        }
        System.out.println();
    }

    // ====================测试代码====================
    // 测试要找的结点在链表中间
    public static void Test1()
    {
        System.out.println("=====Test1 starts:=====");
        ListNode pNode1 = new ListNode(1);
        ListNode pNode2 = new ListNode(2);
        ListNode pNode3 = new ListNode(3);
        ListNode pNode4 = new ListNode(4);
        ListNode pNode5 = new ListNode(5);
        ListNode pNode6 = new ListNode(6);

        pNode1.setM_pNext(pNode2);
        pNode2.setM_pNext(pNode3);
        pNode3.setM_pNext(pNode4);
        pNode4.setM_pNext(pNode5);
        pNode5.setM_pNext(pNode6);

        System.out.println("List: ");
        printList(pNode1);
        ListNode pNode = findKthToTail(pNode1, 2);
        if (pNode == null){
            System.out.println("2th expected result is: " + pNode);
        }else{
            System.out.println("2th expected result is: " + pNode.getM_nValue());
        }
        System.out.println();
    }

    // 测试要找的结点是链表的尾结点
    public static void Test2()
    {
        System.out.println("=====Test2 starts:=====");
        ListNode pNode1 = new ListNode(1);
        ListNode pNode2 = new ListNode(2);
        ListNode pNode3 = new ListNode(3);
        ListNode pNode4 = new ListNode(4);
        ListNode pNode5 = new ListNode(5);
        ListNode pNode6 = new ListNode(6);

        pNode1.setM_pNext(pNode2);
        pNode2.setM_pNext(pNode3);
        pNode3.setM_pNext(pNode4);
        pNode4.setM_pNext(pNode5);
        pNode5.setM_pNext(pNode6);

        System.out.println("List: ");
        printList(pNode1);
        ListNode pNode = findKthToTail(pNode1, 1);
        if (pNode == null){
            System.out.println("1th expected result is: " + pNode);
        }else{
            System.out.println("1th expected result is: " + pNode.getM_nValue());
        }
        System.out.println();
    }

    // 测试要找的结点是链表的头结点
    public static void Test3() {
        System.out.println("=====Test3 starts:=====");
        ListNode pNode1 = new ListNode(1);
        ListNode pNode2 = new ListNode(2);
        ListNode pNode3 = new ListNode(3);
        ListNode pNode4 = new ListNode(4);
        ListNode pNode5 = new ListNode(5);
        ListNode pNode6 = new ListNode(6);

        pNode1.setM_pNext(pNode2);
        pNode2.setM_pNext(pNode3);
        pNode3.setM_pNext(pNode4);
        pNode4.setM_pNext(pNode5);
        pNode5.setM_pNext(pNode6);

        System.out.println("List: ");
        printList(pNode1);
        ListNode pNode = findKthToTail(pNode1, 6);
        if (pNode == null) {
            System.out.println("6th expected result is: " + pNode);
        } else {
            System.out.println("6th expected result is: " + pNode.getM_nValue());
        }
        System.out.println();
    }

    // 测试空链表
    public static void Test4()
    {
        System.out.println("=====Test4 starts:=====");
        ListNode pNode = findKthToTail(null, 100);
        if (pNode == null) {
            System.out.println("100th expected result is: " + pNode);
        } else {
            System.out.println("100th expected result is: " + pNode.getM_nValue());
        }
        System.out.println();
    }

    // 测试输入的第二个参数大于链表的结点总数
    public static void Test5()
    {
        System.out.println("=====Test5 starts:=====");
        ListNode pNode1 = new ListNode(1);
        ListNode pNode2 = new ListNode(2);
        ListNode pNode3 = new ListNode(3);
        ListNode pNode4 = new ListNode(4);
        ListNode pNode5 = new ListNode(5);
        ListNode pNode6 = new ListNode(6);

        pNode1.setM_pNext(pNode2);
        pNode2.setM_pNext(pNode3);
        pNode3.setM_pNext(pNode4);
        pNode4.setM_pNext(pNode5);
        pNode5.setM_pNext(pNode6);

        System.out.println("List: ");
        printList(pNode1);
        ListNode pNode = findKthToTail(pNode1, 7);
        if (pNode == null) {
            System.out.println("7th expected result is: " + pNode);
        } else {
            System.out.println("7th expected result is: " + pNode.getM_nValue());
        }
        System.out.println();
    }

    // 测试输入的第二个参数为0
    public static void Test6()
    {
        System.out.println("=====Test6 starts:=====");
        ListNode pNode1 = new ListNode(1);
        ListNode pNode2 = new ListNode(2);
        ListNode pNode3 = new ListNode(3);
        ListNode pNode4 = new ListNode(4);
        ListNode pNode5 = new ListNode(5);
        ListNode pNode6 = new ListNode(6);

        pNode1.setM_pNext(pNode2);
        pNode2.setM_pNext(pNode3);
        pNode3.setM_pNext(pNode4);
        pNode4.setM_pNext(pNode5);
        pNode5.setM_pNext(pNode6);

        System.out.println("List: ");
        printList(pNode1);
        ListNode pNode = findKthToTail(pNode1, 0);
        if (pNode == null) {
            System.out.println("0th expected result is: " + pNode);
        } else {
            System.out.println("0th expected result is: " + pNode.getM_nValue());
        }
        System.out.println();
    }

    public static void main(String[] args) {
        Test1();
        Test2();
        Test3();
        Test4();
        Test5();
        Test6();
    }
}

 

输出结果：

=====Test1 starts:=====

List: 

1  2  3  4  5  6  

2th expected result is: 5

 

=====Test2 starts:=====

List: 

1  2  3  4  5  6  

1th expected result is: 6

 

=====Test3 starts:=====

List: 

1  2  3  4  5  6  

6th expected result is: 1

 

=====Test4 starts:=====

100th expected result is: null

 

=====Test5 starts:=====

List: 

1  2  3  4  5  6  

7th expected result is: null

 

=====Test6 starts:=====

List: 

1  2  3  4  5  6  

0th expected result is: null