（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：

struct ListNode { int m_nKey; ListNode* m_pNext; };

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

#include <iostream> #include <stack>

struct ListNode { int m_nKey; ListNode* m_pNext; }; void AddToTail(ListNode** pHead, int value) { ListNode* pNew = new ListNode(); pNew->m_nKey = value; pNew->m_pNext = NULL; if (*pHead == NULL) { *pHead = pNew; } else { ListNode* pNode = *pHead; while (pNode->m_pNext != NULL) pNode = pNode->m_pNext; pNode->m_pNext = pNew; } } void PrintLinkedList(const ListNode* head) { if (head == NULL) // 易漏点 return; ListNode* ptr = (ListNode*) head; while (ptr->m_pNext != NULL) { std::cout << ptr->m_nKey << " -> "; ptr = ptr->m_pNext; } std::cout << ptr->m_nKey << std::endl; } void DestroyLinkedList(ListNode** pHead) { if (pHead == NULL || *pHead == NULL) // 易漏点 return; ListNode* pNode = NULL; while (*pHead != NULL) { pNode = *pHead; *pHead = (*pHead)->m_pNext; delete pNode; } } // Iterative method
void PrintListReversingly_Iteratively(const ListNode* pHead) { if (pHead == NULL) return; ListNode* pNode = (ListNode*) pHead; std::stack<ListNode*> nodes; while (pNode != NULL) { nodes.push(pNode); pNode = pNode->m_pNext; } while (!nodes.empty()) { pNode = nodes.top(); nodes.pop(); std::cout << pNode->m_nKey << ", "; } std::cout << std::endl; } // Recursive method
void PrintListReversingly_Recursively(const ListNode* pHead) { if (pHead != NULL) { if (pHead->m_pNext != NULL) { PrintListReversingly_Recursively(pHead->m_pNext); } std::cout << pHead->m_nKey << ", "; } } void unitest() { ListNode* head = NULL; AddToTail(&head, 1); AddToTail(&head, 2); AddToTail(&head, 3); AddToTail(&head, 5); AddToTail(&head, 4); std::cout << "Print forward: "; PrintLinkedList(head); std::cout << "Print reversely iteratively: "; PrintListReversingly_Iteratively(head); std::cout << "Print reversely recursively: "; PrintListReversingly_Recursively(head); // Release memory
    DestroyLinkedList(&head); // 易漏点 } int main() { unitest(); return 0; }

Python 实现

#!/usr/bin/python # -*- coding: utf8 -*-

from __future__ import print_function class ListNode: def __init__(self, value, next_node=None): self.value = value self.next = next_node def add_to_tail(head, value): q = ListNode(value) if head is None: head = q else: p = head while p.next is not None: p = p.next p.next = q return head def print_list_reversely_iteratively(head): p = head stack = [] # Push into stack
    while p is not None: stack.append(p.value) p = p.next # Pop from stack
    while stack: elem = stack.pop() print(elem, end=', ') print('') def print_list_reversely_recursively(head): if head is None: return
    if head.next is not None: print_list_reversely_recursively(head.next) print(head.value, end=', ') def print_linked_list_forward(head): if head is None: print("This linked list is empty!") return p = head while p is not None: print(p.value, end='') if p.next is not None: print(' -> ', end='') p = p.next print('') def unitest(): linked_list = None linked_list = add_to_tail(linked_list, 1) linked_list = add_to_tail(linked_list, 2) linked_list = add_to_tail(linked_list, 3) linked_list = add_to_tail(linked_list, 5) linked_list = add_to_tail(linked_list, 4) print("Print forward: ", end='') print_linked_list_forward(linked_list) print("Print reversely iteratively: ", end='') print_list_reversely_iteratively(linked_list) print("Print reversely recursively: ", end='') print_list_reversely_recursively(linked_list) if __name__ == '__main__': 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)

#pragma once

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

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2. stdafx.h (05_PrintListInReversedOrder/ 目录)

// stdafx.h : include file for standard system include files, // or project specific include files that are used frequently, but // are changed infrequently // 
#pragma once #include "targetver.h" #include <stdio.h> #include <tchar.h>



// TODO: reference additional headers your program requires here

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3. stdafx.cpp (05_PrintListInReversedOrder/ 目录)

// stdafx.cpp : source file that includes just the standard includes // PrintListInReversedOrder.pch will be the pre-compiled header // stdafx.obj will contain the pre-compiled type information
 #include "stdafx.h"

// TODO: reference any additional headers you need in STDAFX.H // and not in this file

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4. PrintListInReversedOrder.cpp

// PrintListInReversedOrder.cpp : Defines the entry point for the console application. //

// 《剑指Offer——名企面试官精讲典型编程题》代码 // 著作权所有者：何海涛
 #include "stdafx.h" #include "..\Utilities\List.h" #include <stack>

void PrintListReversingly_Iteratively(ListNode* pHead) { std::stack<ListNode*> nodes; ListNode* pNode = pHead; while(pNode != NULL) { nodes.push(pNode); pNode = pNode->m_pNext; } while(!nodes.empty()) { pNode = nodes.top(); printf("%d\t", pNode->m_nValue); nodes.pop(); } } void PrintListReversingly_Recursively(ListNode* pHead) { if(pHead != NULL) { if (pHead->m_pNext != NULL) { PrintListReversingly_Recursively(pHead->m_pNext); } printf("%d\t", pHead->m_nValue); } } void Test(ListNode* pHead) { PrintList(pHead); PrintListReversingly_Iteratively(pHead); printf("\n"); PrintListReversingly_Recursively(pHead); } // 1->2->3->4->5
void Test1() { printf("\nTest1 begins.\n"); ListNode* pNode1 = CreateListNode(1); ListNode* pNode2 = CreateListNode(2); ListNode* pNode3 = CreateListNode(3); ListNode* pNode4 = CreateListNode(4); ListNode* pNode5 = CreateListNode(5); ConnectListNodes(pNode1, pNode2); ConnectListNodes(pNode2, pNode3); ConnectListNodes(pNode3, pNode4); ConnectListNodes(pNode4, pNode5); Test(pNode1); DestroyList(pNode1); } // 只有一个结点的链表: 1
void Test2() { printf("\nTest2 begins.\n"); ListNode* pNode1 = CreateListNode(1); Test(pNode1); DestroyList(pNode1); } // 空链表
void Test3() { printf("\nTest3 begins.\n"); Test(NULL); } int _tmain(int argc, _TCHAR* argv[]) { Test1(); Test2(); Test3(); return 0; }

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5. targetver.h (Utilities/ 目录)

#pragma once

// The following macros define the minimum required platform. The minimum required platform // is the earliest version of Windows, Internet Explorer etc. that has the necessary features to run // your application. The macros work by enabling all features available on platform versions up to and // including the version specified. // Modify the following defines if you have to target a platform prior to the ones specified below. // Refer to MSDN for the latest info on corresponding values for different platforms.
#ifndef WINVER                          // Specifies that the minimum required platform is Windows Vista.
#define WINVER 0x0600           // Change this to the appropriate value to target other versions of Windows.
#endif #ifndef _WIN32_WINNT // Specifies that the minimum required platform is Windows Vista.
#define _WIN32_WINNT 0x0600     // Change this to the appropriate value to target other versions of Windows.
#endif #ifndef _WIN32_WINDOWS // Specifies that the minimum required platform is Windows 98.
#define _WIN32_WINDOWS 0x0410 // Change this to the appropriate value to target Windows Me or later.
#endif #ifndef _WIN32_IE // Specifies that the minimum required platform is Internet Explorer 7.0.
#define _WIN32_IE 0x0700        // Change this to the appropriate value to target other versions of IE.
#endif

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6. stdafx.h (Utilities/ 目录)

// stdafx.h : include file for standard system include files, // or project specific include files that are used frequently, but // are changed infrequently // 
#pragma once #include "targetver.h"

#define WIN32_LEAN_AND_MEAN             // Exclude rarely-used stuff from Windows headers
// Windows Header Files:
#include <windows.h> #include <stdio.h>


// TODO: reference additional headers your program requires here

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7. stdafx.cpp (Utilities/ 目录)

// stdafx.cpp : source file that includes just the standard includes // Utilities.pch will be the pre-compiled header // stdafx.obj will contain the pre-compiled type information
 #include "stdafx.h"

// TODO: reference any additional headers you need in STDAFX.H // and not in this file

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8. List.h 

// 《剑指Offer——名企面试官精讲典型编程题》代码 // 著作权所有者：何海涛

struct ListNode { int m_nValue; ListNode* m_pNext; }; __declspec( dllexport ) ListNode* CreateListNode(int value); __declspec( dllexport ) void ConnectListNodes(ListNode* pCurrent, ListNode* pNext); __declspec( dllexport ) void PrintListNode(ListNode* pNode); __declspec( dllexport ) void PrintList(ListNode* pHead); __declspec( dllexport ) void DestroyList(ListNode* pHead); __declspec( dllexport ) void AddToTail(ListNode** pHead, int value); __declspec( dllexport ) void RemoveNode(ListNode** pHead, int value);

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9. List.cpp 

// Utilities.cpp : Defines the exported functions for the DLL application. //

// 《剑指Offer——名企面试官精讲典型编程题》代码 // 著作权所有者：何海涛
 #include "stdafx.h" #include "list.h" #include <stdio.h> #include <stdlib.h> ListNode* CreateListNode(int value) { ListNode* pNode = new ListNode(); pNode->m_nValue = value; pNode->m_pNext = NULL; return pNode; } void ConnectListNodes(ListNode* pCurrent, ListNode* pNext) { if(pCurrent == NULL) { printf("Error to connect two nodes.\n"); exit(1); } pCurrent->m_pNext = pNext; } void PrintListNode(ListNode* pNode) { if(pNode == NULL) { printf("The node is NULL\n"); } else { printf("The key in node is %d.\n", pNode->m_nValue); } } void PrintList(ListNode* pHead) { printf("PrintList starts.\n"); ListNode* pNode = pHead; while(pNode != NULL) { printf("%d\t", pNode->m_nValue); pNode = pNode->m_pNext; } printf("\nPrintList ends.\n"); } void DestroyList(ListNode* pHead) { ListNode* pNode = pHead; while(pNode != NULL) { pHead = pHead->m_pNext; delete pNode; pNode = pHead; } } void AddToTail(ListNode** pHead, int value) { ListNode* pNew = new ListNode(); pNew->m_nValue = value; pNew->m_pNext = NULL; if(*pHead == NULL) { *pHead = pNew; } else { ListNode* pNode = *pHead; while(pNode->m_pNext != NULL) pNode = pNode->m_pNext; pNode->m_pNext = pNew; } } void RemoveNode(ListNode** pHead, int value) { if(pHead == NULL || *pHead == NULL) return; ListNode* pToBeDeleted = NULL; if((*pHead)->m_nValue == value) { pToBeDeleted = *pHead; *pHead = (*pHead)->m_pNext; } else { ListNode* pNode = *pHead; while(pNode->m_pNext != NULL && pNode->m_pNext->m_nValue != value) pNode = pNode->m_pNext; if(pNode->m_pNext != NULL && pNode->m_pNext->m_nValue == value) { pToBeDeleted = pNode->m_pNext; pNode->m_pNext = pNode->m_pNext->m_pNext; } } if(pToBeDeleted != NULL) { delete pToBeDeleted; pToBeDeleted = NULL; } }

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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.