检查字符串是否为回文时出错

Question

I'm trying to make a C program that checks whether a sequence of characters is palindrome or not. I must use a stack and a recursive function.

The first number given as input is the length of the string. The program works correctly with even lengths, but doesn't recognize palindromeness of odd strings. I have been trying to find the mistake for at least an hour but no luck.

This is the code.

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

struct c {
    char cha;
    struct c *nextPtr;
};

typedef struct c Character;

void push(Character **headPtr, char c) {
    Character *newC = malloc(sizeof(Character));

    if(newC == NULL) {
        puts("Insufficient memory.");
    }
    newC->cha = c;
    newC->nextPtr = *headPtr;
    *headPtr = newC;
}

char pop(Character **headPtr) {
    if(*headPtr != NULL) {
        Character *tempPtr = *headPtr;
        char c = (*headPtr)->cha;
        *headPtr = (*headPtr)->nextPtr;
        free(tempPtr);
        return c;
    }
}

void isPalindrome(int stringLength, int charNum) {
    static Character *headPtr = NULL;

    if(stringLength == 1) {
        puts("Palindrome");
        exit(0);
    }
    if(charNum < (stringLength/2)) {
        char c = getchar();
        push(&headPtr, c);
        return isPalindrome(stringLength, charNum+1);
    }
    if(stringLength % 2) {
        getchar();
    }

    char d = getchar();
    char e = pop(&headPtr);
    if(d != e) {
        puts("Not palindrome");
        exit(0);
    }
    return;
}

int main() {
    int n;

    scanf("%d", &n);
    while(getchar()!='\n');
    isPalindrome(n, 0);

    puts("Palindrome.");
    return 0;
}

You can try it yourself by just inputting an odd number, then enter, than a sequence of characters. The program will not recognize if it's palindrome and always say it's not or even do weirder stuff like not outputting anything at all and waiting for more input.

I really don't know what I'm missing.

Answer 1

Never mind, I found the solution by myself.

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

int isPal = 1;

struct c {
    char cha;
    struct c *nextPtr;
};

typedef struct c Character;

void push(Character **headPtr, char c) {
    Character *newC = malloc(sizeof(Character));

    if(newC == NULL) {
        puts("Insufficient memory.");
        return;
    }
    newC->cha = c;
    newC->nextPtr = *headPtr;
    *headPtr = newC;
}

char pop(Character **headPtr) {
    if(*headPtr != NULL) {
        Character *tempPtr = *headPtr;
        char c = (*headPtr)->cha;
        *headPtr = (*headPtr)->nextPtr;
        free(tempPtr);
        return c;
    }
}

void isPalindrome(int stringLength, int charNum) {
    if(!isPal) return;

    static Character *stack = NULL;

    if(charNum < (stringLength/2)) {
        char c = getchar();
        push(&stack, c);
        isPalindrome(stringLength, charNum+1);
    } else if(stringLength % 2) {
        getchar();
    }

    char d = getchar();
    char e = pop(&stack);
    if(d != '\n' && d != e) isPal = 0;
}

int main() {
    int n;

    if(scanf("%d", &n) != 1 || n < 0) {
        puts("Incorrect input.");
        return 0;
    }
    while(getchar()!='\n');
    if(n>1) isPalindrome(n, 0);

    if(isPal) puts("Palindrome.");
    else puts("Not palindrome.");
    return 0;
}