递归函数查找回文

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

Something like this will work ?

Something like this will work ?

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

bool palindrom_helper(char* str, int first, int last)
{
    if(first >= last)
        return true;
    if (str[first] != str[last]) 
        return false;
    return (palindrom(str, first+1, last-1));

}

bool palindrom(char* str)
{
    return palindrom_helper(str, 0, strlen(str)-1);
}

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;
}

you should use the strlen function only once. so you cannot use it inside the function that's being called recursively.

what i did here is to initialize first and last to 0 and len-1 and then recurse with (first+1, last-1).

if the function finds even one pair of unmatching letters, it'd return false. else it'd continue until they reach the center together (odd length string) or cross each other (even length string) and then return true (because that'd mean they did not see any unmatching letter on their path)

Also, i dont understand what you mean by single transition in the recursive function ?

Answer 2

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