Recursive function finding Palindrome

Question

May be you can tell me the way, that I can start with, at least. I can use only C language. The task have very specific limitations and I can't break them in any way. The task is:

• Write recursive function that checks if string is Palindrome.
• Can use strlen() only once in function.
• Can't use any loops or functions based on loops.
• Can use only one transition in recursive function.
• Can change the string, but only if it will come back in the end of function.
• Declaration of function is: int palindrom(char* str);

• I started to write, but have no ideas anymore:

  #define _CRT_SECURE_NO_WARNINGS
  #include <stdio.h>
  #include <string.h>
  
  int palindrom(char* str)
  {
      int len = strlen(str);
      if (str[0] != str[len - 1]) return 0;
  
  }
  
  int main(void)
  {
      char string1[] = "ROTATOR";
      char string2[] = "8536358";
      char string3[] = "Palindrome";
      if (palindrom(string1)) printf("%s is Palindrome\n", string1);
      else printf("%s is not Palindrome\n", string1);
      if (palindrom(string2)) printf("%s is Palindrome\n", string2);
      else printf("%s is not Palindrome\n", string2);
      if (palindrom(string3)) printf("%s is Palindrome\n", string3);
      else printf("%s is not Palindrome\n", string3);
      return 0;
  }
  

Answer 1

The clue to how to solve is in the restriction: Can change the string, but only if it will come back in the end of function. That condition is checked by printing the result.

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

int palindrom(char* str)
{
    size_t len = strlen(str);
    int res;
    if(len < 2) {
        return 1;                   // cannot shorten: must be success
    }
    if(str[0] != str[len - 1]) {    // make palindrome test
        return 0;
    }

    str[len - 1] = '\0';            // shorten the string at the back
    res = palindrom(str + 1);       // recurse woth string shortened at the front
    str[len - 1] = str[0];          // replace last char (we know it's the same)
    return res;
}

int main(void)
{
    char string1[] = "ROTATOR";
    char string2[] = "8536358";
    char string3[] = "Palindrome";
    char string4[] = "A";
    char *wrd[] = { "not ", "" };

    printf("%s is %sa Palindrome\n", string1, wrd[ palindrom(string1) ]);
    printf("%s is %sa Palindrome\n", string2, wrd[ palindrom(string2) ]);
    printf("%s is %sa Palindrome\n", string3, wrd[ palindrom(string3) ]);
    printf("%s is %sa Palindrome\n", string4, wrd[ palindrom(string4) ]);

    return 0;
}

Program output:

ROTATOR is a Palindrome
8536358 is a Palindrome
Palindrome is not a Palindrome
A is a Palindrome