使用堆栈检查字符串是否为回文结构

Question

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

typedef struct node
{
    char data;
    struct node *link;
}StackNode;

void insertData(StackNode **);
void push(StackNode **, char);
void checkData(StackNode **);
bool pop(StackNode **,char *);

char sent[20] = "";

void main()
{
   StackNode *stackTop;
   insertData(&stackTop);
   checkData(&stackTop);
   printf("\n");
   return;
}

void insertData(StackNode **stackTop)
{
    char c;
    int len;

    printf("Enter the Sentence\n");
    while( ( ( c = getchar() ) != '\n'))
    {   
        if( ( ( c>='a' &&c<='z') || (c>='A' && c<='Z')))
        {
            if((c>='A' && c<='Z'))
            {
                int rem;
                rem = c-'A';
                c='a' + rem;
            }
            push(stackTop,c);
            len = strlen(sent);
            sent[len++]=c;
            sent[len]='\0';
        }
    }
    printf("Letters are %s\n\n",sent);
}

void push(StackNode **stackTop,char c)
{
    StackNode *pNew;
    pNew = (StackNode*) malloc(sizeof(StackNode));
    if(!pNew)
    {
        printf("Error 100:Out of memory\n");
        exit(100);
    }
    pNew->data = c;
    pNew->link = *stackTop;
    *stackTop = pNew;
}

void checkData(StackNode **stackTop)
{
    char c;
    int i=0;
    while(pop(stackTop,&c))
    {
        if( c !=sent[i++])
        {
            printf("Not palindrome");
            return;
        }
    }
    printf("Palindrome");
}

bool pop(StackNode **stackTop,char *c)
{
    StackNode *pNew;
    pNew = *stackTop;
    if(pNew == NULL)
        return false;
    *c = pNew->data;
    *stackTop = pNew->link;
    printf("char poped %c\n",*c);
    free(pNew);
    return true;
}

I am able to pop each letter, after that the exe file is not working, I think I'm unable to check if the letters are same in reverse order. please help

I've been working all day to write this program,but I'm stuck at this point.

Answer 1

Are you allowed to use the implicit call stack?

int is_pal(const char *begin, const char *end)
{
    if (*begin != *end)
        return 0;

    if ((end - begin) < 2)
        return *begin == *end;

    return is_pal(begin + 1, end - 1);
}

Answer 2

When programming in C, you should always initialize pointers to NULL . Because you didn't initialize the StackNode *stackTop (in main) it is pointing to garbage and is not null. So when you are checking for a palindrome, your pop method just keeps going off the end of the stack and never returns false because it never hits a null. StackNode *stackTop = NULL; should fix your problem.

Answer 3

I am not sure why you want to do this, but this is one way

put everything on the stack

and then pop everything comparing with the elements of the string from the start. something like this

for(int i = 0; i < strlen(s); i++) push(s[i]);

for(int i = 0; i < strlen(s); i++) if (s[i] == pop()) continue;