剑指Offer系列---(7)从尾到头打印链表

1.题目描述：

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。链表结点的定义如下：

struct ListNode{

int m_nKey;

ListNode *m_pNext;

};

2.源代码：

A1.用栈实现的“从尾到头打印链表”程序代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>

#ifndef ERROR
#define ERROR (0)
#endif
#ifndef OK
#define OK   (!ERROR)
#endif

#define STACK_INIT_SIZE 1
#define STACKINCREMENT 10

typedef int ElemType;
typedef struct Node{
ElemType data;
struct Node *next;
}Node,*pNode;

typedef int SElemType;
typedef struct SqStack{
SElemType  *base;
SElemType  *top;
int  stacksize;
}SqStack,*pStack;

pStack S;

pStack InitStack(pStack S)
{
S = (pStack)malloc(STACK_INIT_SIZE * sizeof(SElemType));
if (S == NULL) {
return ERROR;
}
S->base = (SElemType *)S;
S->top = S->base;
S->stacksize = STACK_INIT_SIZE;

return S;
}

pStack Push(pStack S,SElemType e)
{
if((S->top - S->base) >= S->stacksize){
S->base = (SElemType*)realloc(S, (S->stacksize +STACKINCREMENT)*sizeof(SElemType));
if (S->base == NULL) {
return ERROR;
}
S->top = S->base + S->stacksize;
S->stacksize += STACKINCREMENT;
}
*S->top++ = e;
return S;
}

SElemType Pop(pStack S)
{
return *(--S->top);
}

pNode CreateList()
{
ElemType val;
pNode pHead = NULL;
pNode pCur = NULL;

do{
scanf("%d",&val);
if(val != -1){
pNode pNew = (pNode)malloc(sizeof(Node));
if (pNew == NULL)
exit(EXIT_FAILURE);
pNew->data = val;
pNew->next = NULL;

if (pHead == NULL) {
pHead = pNew;
pCur = pHead;
}else{
pCur->next = pNew;
pCur = pCur->next;
}
}
}while (val != -1);
return pHead;
}

void DestroyList(pNode pHead)
{
if(pHead == NULL)
return;
pNode p = NULL;
while (pHead != NULL) {
p = pHead->next;
free(pHead);
pHead = p;
}
}

void PrintListReverse(pNode pHead)
{
if(pHead == NULL)
return;
while (pHead != NULL) {
Push(S, pHead->data);
pHead = pHead->next;
}
while (S->top != S->base) {
printf("%d\n",Pop(S));
}
}

int main(int argc, char ** argv)
{
S = InitStack(S);

pNode pHead = CreateList();
PrintListReverse(pHead);
DestroyList(pHead);
return 0;
}

A2.用递归实现的“从尾到头打印链表”程序代码如下：

#include <stdio.h>
#include <stdlib.h>

typedef int ElemType;

typedef struct Node{
ElemType  data;
struct Node *next;
}Node,*pNode;

void PrintListReverse(pNode pHead)
{
if(pHead == NULL)
return;
if(pHead->next != NULL)
PrintListReverse(pHead->next);
printf("%d\n",pHead->data);
}

pNode CreateList()
{
ElemType val;
pNode pHead = NULL;
pNode pCur = NULL;

do{
scanf("%d",&val);
if(val != -1){
pNode pNew = (pNode)malloc(sizeof(Node));
if(pNew == NULL)
exit(EXIT_FAILURE);
pNew->data = val;
pNew->next = NULL;

if(pHead == NULL){
pHead = pNew;
pCur = pHead;
}else{
pCur->next = pNew;
pCur = pCur->next;
}
}
}while (val != -1);
return pHead;
}

void DestroyList(pNode pHead)
{
if (pHead == NULL) {
return;
}
pNode p = NULL;
while (pHead != NULL) {
p = pHead->next;
free(pHead);
pHead = p;
}
}

int main(int argc,char **argv)
{
pNode pHead = CreateList();
PrintListReverse(pHead);
DestroyList(pHead);
return 0;
}

3.小结：

1）对于反向输出时，应该考虑它的特性，选择数据结构类型来实现。一定要搞清楚各种数据结构类型的特点；

2）对于栈能实现的例子，一般要想到也可以用递归来完成。递归的缺点就是递归层级很深时，可能导致函数调用栈溢出；

3）在面试过程中，可以试着问面试官一些关于题目的信息，比如上面的从尾到头打印链表，是否允许对输入的数据做修改。