（说明：本博客中的题目、题目详细说明及参考代码均摘自 “何海涛《剑指Offer：名企面试官精讲典型编程题》2012年”）

题目

输入一个链表的头结点， 从尾到头反过来打印出每个结点的值。

进一步详细说明：
不允许在打印时修改链表的结构。链表结点可定义为：

  1. struct ListNode
  2. {
  3.     int m_nKey;
  4.     ListNode* m_pNext;
  5. };

算法设计思想

正常情况，遍历链表都是从前到后的，即从头到尾。如果从尾到头打印链表元素，可以借助栈的 “后入先出” （Last in, First out）的性质，在正向遍历时将链表元素依次压栈，当到达链表末尾时，再依次弹出并打印。

具体实现可以采用两种方法：迭代和递归。正如书中所说，“递归在本质上就是一个栈结构”。递归实现，从理论上，完全可以利用栈结构转换为非递归实现，即迭代方法。

C++ 实现

  1. #include <iostream>
  2. #include <stack>
  3.  
  4. struct ListNode
  5. {
  6.     int  m_nKey;
  7.     ListNode* m_pNext;
  8. };
  9.  
  10. void AddToTail(ListNode** pHead, int value)
  11. {
  12.     ListNode* pNew = new ListNode();
  13.     pNew->m_nKey = value;
  14.     pNew->m_pNext = NULL;
  15.  
  16.     if (*pHead == NULL)
  17.     {
  18.         *pHead = pNew;
  19.     }
  20.     else
  21.     {
  22.         ListNode* pNode = *pHead;
  23.         while (pNode->m_pNext != NULL)
  24.             pNode = pNode->m_pNext;
  25.         pNode->m_pNext = pNew;
  26.     }
  27. }
  28.  
  29. void PrintLinkedList(const ListNode* head)
  30. {
  31.     if (head == NULL)  // 易漏点
  32.         return;
  33.  
  34.     ListNode* ptr = (ListNode*) head;
  35.     while (ptr->m_pNext != NULL)
  36.     {
  37.         std::cout << ptr->m_nKey << " -> ";
  38.         ptr = ptr->m_pNext;
  39.     }
  40.  
  41.     std::cout << ptr->m_nKey << std::endl;
  42. }
  43.  
  44. void DestroyLinkedList(ListNode** pHead)
  45. {
  46.     if (pHead == NULL || *pHead == NULL)  // 易漏点
  47.         return;
  48.  
  49.     ListNode* pNode = NULL;
  50.     while (*pHead != NULL)
  51.     {
  52.         pNode = *pHead;
  53.         *pHead = (*pHead)->m_pNext;
  54.         delete pNode;
  55.     }
  56. }
  57.  
  58. // Iterative method
  59. void PrintListReversingly_Iteratively(const ListNode* pHead)
  60. {
  61.     if (pHead == NULL)
  62.         return;
  63.  
  64.     ListNode* pNode = (ListNode*) pHead;
  65.     std::stack<ListNode*> nodes;
  66.  
  67.     while (pNode != NULL)
  68.     {
  69.         nodes.push(pNode);
  70.         pNode = pNode->m_pNext;
  71.     }
  72.  
  73.     while (!nodes.empty())
  74.     {
  75.         pNode = nodes.top();
  76.         nodes.pop();
  77.         std::cout << pNode->m_nKey << ", ";
  78.     }
  79.     std::cout << std::endl;
  80. }
  81.  
  82. // Recursive method
  83. void PrintListReversingly_Recursively(const ListNode* pHead)
  84. {
  85.     if (pHead != NULL)
  86.     {
  87.         if (pHead->m_pNext != NULL)
  88.         {
  89.             PrintListReversingly_Recursively(pHead->m_pNext);
  90.         }
  91.  
  92.         std::cout << pHead->m_nKey << ", ";
  93.     }
  94. }
  95.  
  96. void unitest()
  97. {
  98.     ListNode* head = NULL;
  99.  
  100.     AddToTail(&head, );
  101.     AddToTail(&head, );
  102.     AddToTail(&head, );
  103.     AddToTail(&head, );
  104.     AddToTail(&head, );
  105.  
  106.     std::cout << "Print forward: ";
  107.     PrintLinkedList(head);
  108.     std::cout << "Print reversely iteratively: ";
  109.     PrintListReversingly_Iteratively(head);
  110.     std::cout << "Print reversely recursively: ";
  111.     PrintListReversingly_Recursively(head);
  112.  
  113.     // Release memory
  114.     DestroyLinkedList(&head);   // 易漏点
  115. }
  116.  
  117. int main()
  118. {
  119.     unitest();
  120.  
  121.     return ;
  122. }

Python 实现

  1. #!/usr/bin/python
  2. # -*- coding: utf8 -*-
  3.  
  4. from __future__ import print_function
  5.  
  6. class ListNode:
  7.     def __init__(self, value, next_node=None):
  8.         self.value = value
  9.         self.next = next_node
  10.  
  11. def add_to_tail(head, value):
  12.     q = ListNode(value)
  13.  
  14.     if head is None:
  15.         head = q
  16.     else:
  17.         p = head
  18.         while p.next is not None:
  19.             p = p.next
  20.         p.next = q
  21.  
  22.     return head 
  23.  
  24. def print_list_reversely_iteratively(head):
  25.     p = head
  26.     stack = []
  27.     # Push into stack
  28.     while p is not None:
  29.         stack.append(p.value)
  30.         p = p.next
  31.     # Pop from stack
  32.     while stack:
  33.         elem = stack.pop()
  34.         print(elem, end=', ')
  35.     print('')
  36.  
  37. def print_list_reversely_recursively(head):
  38.     if head is None:
  39.         return
  40.     if head.next is not None:
  41.         print_list_reversely_recursively(head.next)
  42.     print(head.value, end=', ')
  43.  
  44. def print_linked_list_forward(head):
  45.     if head is None:
  46.         print("This linked list is empty!")
  47.         return
  48.  
  49.     p = head
  50.     while p is not None:
  51.         print(p.value, end='')
  52.         if p.next is not None:
  53.             print(' -> ', end='')
  54.         p = p.next
  55.     print('')
  56.  
  57. def unitest():
  58.     linked_list = None
  59.     linked_list = add_to_tail(linked_list, 1)
  60.     linked_list = add_to_tail(linked_list, 2)
  61.     linked_list = add_to_tail(linked_list, 3)
  62.     linked_list = add_to_tail(linked_list, 5)
  63.     linked_list = add_to_tail(linked_list, 4)
  64.     print("Print forward: ", end='')
  65.     print_linked_list_forward(linked_list)
  66.     print("Print reversely iteratively: ", end='')
  67.     print_list_reversely_iteratively(linked_list)
  68.     print("Print reversely recursively: ", end='')
  69.     print_list_reversely_recursively(linked_list)
  70.  
  71. if __name__ == '__main__':
  72.     unitest()

注：使用 Python 利用函数建立链表时，需要注意函数参数的值传递和引用传递，此为易错点。

参考代码

1. targetver.h (05_PrintListInReversedOrder/ 目录)

  1. #pragma once
  2.  
  3. // The following macros define the minimum required platform.  The minimum required platform
  4. // is the earliest version of Windows, Internet Explorer etc. that has the necessary features to run
  5. // your application.  The macros work by enabling all features available on platform versions up to and
  6. // including the version specified.
  7.  
  8. // Modify the following defines if you have to target a platform prior to the ones specified below.
  9. // Refer to MSDN for the latest info on corresponding values for different platforms.
  10. #ifndef _WIN32_WINNT            // Specifies that the minimum required platform is Windows Vista.
  11. #define _WIN32_WINNT 0x0600     // Change this to the appropriate value to target other versions of Windows.
  12. #endif

2. stdafx.h (05_PrintListInReversedOrder/ 目录)

  1. // stdafx.h : include file for standard system include files,
  2. // or project specific include files that are used frequently, but
  3. // are changed infrequently
  4. //
  5.  
  6. #pragma once
  7.  
  8. #include "targetver.h"
  9.  
  10. #include <stdio.h>
  11. #include <tchar.h>
  12.  
  13. // TODO: reference additional headers your program requires here

3. stdafx.cpp (05_PrintListInReversedOrder/ 目录)

  1. // stdafx.cpp : source file that includes just the standard includes
  2. // PrintListInReversedOrder.pch will be the pre-compiled header
  3. // stdafx.obj will contain the pre-compiled type information
  4.  
  5. #include "stdafx.h"
  6.  
  7. // TODO: reference any additional headers you need in STDAFX.H
  8. // and not in this file

4. PrintListInReversedOrder.cpp

  1. // PrintListInReversedOrder.cpp : Defines the entry point for the console application.
  2. //
  3.  
  4. // 《剑指Offer——名企面试官精讲典型编程题》代码
  5. // 著作权所有者：何海涛
  6.  
  7. #include "stdafx.h"
  8. #include "..\Utilities\List.h"
  9. #include <stack>
  10.  
  11. void PrintListReversingly_Iteratively(ListNode* pHead)
  12. {
  13.     std::stack<ListNode*> nodes;
  14.  
  15.     ListNode* pNode = pHead;
  16.     while(pNode != NULL)
  17.     {
  18.         nodes.push(pNode);
  19.         pNode = pNode->m_pNext;
  20.     }
  21.  
  22.     while(!nodes.empty())
  23.     {
  24.         pNode = nodes.top();
  25.         printf("%d\t", pNode->m_nValue);
  26.         nodes.pop();
  27.     }
  28. }
  29.  
  30. void PrintListReversingly_Recursively(ListNode* pHead)
  31. {
  32.     if(pHead != NULL)
  33.     {
  34.         if (pHead->m_pNext != NULL)
  35.         {
  36.             PrintListReversingly_Recursively(pHead->m_pNext);
  37.         }
  38.  
  39.         printf("%d\t", pHead->m_nValue);
  40.     }
  41. }
  42.  
  43. void Test(ListNode* pHead)
  44. {
  45.     PrintList(pHead);
  46.     PrintListReversingly_Iteratively(pHead);
  47.     printf("\n");
  48.     PrintListReversingly_Recursively(pHead);
  49. }
  50.  
  51. // 1->2->3->4->5
  52. void Test1()
  53. {
  54.     printf("\nTest1 begins.\n");
  55.  
  56.     ListNode* pNode1 = CreateListNode();
  57.     ListNode* pNode2 = CreateListNode();
  58.     ListNode* pNode3 = CreateListNode();
  59.     ListNode* pNode4 = CreateListNode();
  60.     ListNode* pNode5 = CreateListNode();
  61.  
  62.     ConnectListNodes(pNode1, pNode2);
  63.     ConnectListNodes(pNode2, pNode3);
  64.     ConnectListNodes(pNode3, pNode4);
  65.     ConnectListNodes(pNode4, pNode5);
  66.  
  67.     Test(pNode1);
  68.  
  69.     DestroyList(pNode1);
  70. }
  71.  
  72. // 只有一个结点的链表: 1
  73. void Test2()
  74. {
  75.     printf("\nTest2 begins.\n");
  76.  
  77.     ListNode* pNode1 = CreateListNode();
  78.  
  79.     Test(pNode1);
  80.  
  81.     DestroyList(pNode1);
  82. }
  83.  
  84. // 空链表
  85. void Test3()
  86. {
  87.     printf("\nTest3 begins.\n");
  88.  
  89.     Test(NULL);
  90. }
  91.  
  92. int _tmain(int argc, _TCHAR* argv[])
  93. {
  94.     Test1();
  95.     Test2();
  96.     Test3();
  97.  
  98.     return ;
  99. }

5. targetver.h (Utilities/ 目录)

  1. #pragma once
  2.  
  3. // The following macros define the minimum required platform.  The minimum required platform
  4. // is the earliest version of Windows, Internet Explorer etc. that has the necessary features to run
  5. // your application.  The macros work by enabling all features available on platform versions up to and
  6. // including the version specified.
  7.  
  8. // Modify the following defines if you have to target a platform prior to the ones specified below.
  9. // Refer to MSDN for the latest info on corresponding values for different platforms.
  10. #ifndef WINVER                          // Specifies that the minimum required platform is Windows Vista.
  11. #define WINVER 0x0600           // Change this to the appropriate value to target other versions of Windows.
  12. #endif
  13.  
  14. #ifndef _WIN32_WINNT            // Specifies that the minimum required platform is Windows Vista.
  15. #define _WIN32_WINNT 0x0600     // Change this to the appropriate value to target other versions of Windows.
  16. #endif
  17.  
  18. #ifndef _WIN32_WINDOWS          // Specifies that the minimum required platform is Windows 98.
  19. #define _WIN32_WINDOWS 0x0410 // Change this to the appropriate value to target Windows Me or later.
  20. #endif
  21.  
  22. #ifndef _WIN32_IE                       // Specifies that the minimum required platform is Internet Explorer 7.0.
  23. #define _WIN32_IE 0x0700        // Change this to the appropriate value to target other versions of IE.
  24. #endif

6. stdafx.h (Utilities/ 目录)

  1. // stdafx.h : include file for standard system include files,
  2. // or project specific include files that are used frequently, but
  3. // are changed infrequently
  4. //
  5.  
  6. #pragma once
  7.  
  8. #include "targetver.h"
  9.  
  10. #define WIN32_LEAN_AND_MEAN             // Exclude rarely-used stuff from Windows headers
  11. // Windows Header Files:
  12. #include <windows.h>
  13. #include <stdio.h>
  14.  
  15. // TODO: reference additional headers your program requires here

7. stdafx.cpp (Utilities/ 目录)

  1. // stdafx.cpp : source file that includes just the standard includes
  2. // Utilities.pch will be the pre-compiled header
  3. // stdafx.obj will contain the pre-compiled type information
  4.  
  5. #include "stdafx.h"
  6.  
  7. // TODO: reference any additional headers you need in STDAFX.H
  8. // and not in this file

8. List.h

  1. // 《剑指Offer——名企面试官精讲典型编程题》代码
  2. // 著作权所有者：何海涛
  3.  
  4. struct ListNode
  5. {
  6.     int       m_nValue;
  7.     ListNode* m_pNext;
  8. };
  9.  
  10. __declspec( dllexport ) ListNode* CreateListNode(int value);
  11. __declspec( dllexport ) void ConnectListNodes(ListNode* pCurrent, ListNode* pNext);
  12. __declspec( dllexport ) void PrintListNode(ListNode* pNode);
  13. __declspec( dllexport ) void PrintList(ListNode* pHead);
  14. __declspec( dllexport ) void DestroyList(ListNode* pHead);
  15. __declspec( dllexport ) void AddToTail(ListNode** pHead, int value);
  16. __declspec( dllexport ) void RemoveNode(ListNode** pHead, int value);

9. List.cpp

  1. // Utilities.cpp : Defines the exported functions for the DLL application.
  2. //
  3.  
  4. // 《剑指Offer——名企面试官精讲典型编程题》代码
  5. // 著作权所有者：何海涛
  6.  
  7. #include "stdafx.h"
  8. #include "list.h"
  9. #include <stdio.h>
  10. #include <stdlib.h>
  11.  
  12. ListNode* CreateListNode(int value)
  13. {
  14.     ListNode* pNode = new ListNode();
  15.     pNode->m_nValue = value;
  16.     pNode->m_pNext = NULL;
  17.  
  18.     return pNode;
  19. }
  20.  
  21. void ConnectListNodes(ListNode* pCurrent, ListNode* pNext)
  22. {
  23.     if(pCurrent == NULL)
  24.     {
  25.         printf("Error to connect two nodes.\n");
  26.         exit();
  27.     }
  28.  
  29.     pCurrent->m_pNext = pNext;
  30. }
  31.  
  32. void PrintListNode(ListNode* pNode)
  33. {
  34.     if(pNode == NULL)
  35.     {
  36.         printf("The node is NULL\n");
  37.     }
  38.     else
  39.     {
  40.         printf("The key in node is %d.\n", pNode->m_nValue);
  41.     }
  42. }
  43.  
  44. void PrintList(ListNode* pHead)
  45. {
  46.     printf("PrintList starts.\n");
  47.  
  48.     ListNode* pNode = pHead;
  49.     while(pNode != NULL)
  50.     {
  51.         printf("%d\t", pNode->m_nValue);
  52.         pNode = pNode->m_pNext;
  53.     }
  54.  
  55.     printf("\nPrintList ends.\n");
  56. }
  57.  
  58. void DestroyList(ListNode* pHead)
  59. {
  60.     ListNode* pNode = pHead;
  61.     while(pNode != NULL)
  62.     {
  63.         pHead = pHead->m_pNext;
  64.         delete pNode;
  65.         pNode = pHead;
  66.     }
  67. }
  68.  
  69. void AddToTail(ListNode** pHead, int value)
  70. {
  71.     ListNode* pNew = new ListNode();
  72.     pNew->m_nValue = value;
  73.     pNew->m_pNext = NULL;
  74.  
  75.     if(*pHead == NULL)
  76.     {
  77.         *pHead = pNew;
  78.     }
  79.     else
  80.     {
  81.         ListNode* pNode = *pHead;
  82.         while(pNode->m_pNext != NULL)
  83.             pNode = pNode->m_pNext;
  84.  
  85.         pNode->m_pNext = pNew;
  86.     }
  87. }
  88.  
  89. void RemoveNode(ListNode** pHead, int value)
  90. {
  91.     if(pHead == NULL || *pHead == NULL)
  92.         return;
  93.  
  94.     ListNode* pToBeDeleted = NULL;
  95.     if((*pHead)->m_nValue == value)
  96.     {
  97.         pToBeDeleted = *pHead;
  98.         *pHead = (*pHead)->m_pNext;
  99.     }
  100.     else
  101.     {
  102.         ListNode* pNode = *pHead;
  103.         while(pNode->m_pNext != NULL && pNode->m_pNext->m_nValue != value)
  104.             pNode = pNode->m_pNext;
  105.  
  106.         if(pNode->m_pNext != NULL && pNode->m_pNext->m_nValue == value)
  107.         {
  108.             pToBeDeleted = pNode->m_pNext;
  109.             pNode->m_pNext = pNode->m_pNext->m_pNext;
  110.         }
  111.     }
  112.  
  113.     if(pToBeDeleted != NULL)
  114.     {
  115.         delete pToBeDeleted;
  116.         pToBeDeleted = NULL;
  117.     }
  118. }

10. 参考代码下载

项目 05_PrintListInReversedOrder 下载： 百度网盘

何海涛《剑指Offer：名企面试官精讲典型编程题》 所有参考代码下载：百度网盘

参考资料

[1]  何海涛. 剑指 Offer：名企面试官精讲典型编程题 [M]. 北京：电子工业出版社，2012. 49-53.

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