#include <stdio.h>

#include <stdlib.h>

int strlen(char *s)

{

char *p = s;

while (*p)

p++;

return p - s;

}

char * strcpy(char to[], char from[])

{

char * t = to;

(*to++ = *from++);

return t;

}

// array version

char * strrev1(char a[])

{

int i, j;

char temp;

for (i=0, j=strlen(a)-1 ; i<j ; i++, j--) {

temp = a[i];

a[i] = a[j];

a[j] = temp;

}

/*

int len = strlen(a);

for (i=0 ; i<len/2 ; i++) {

temp = a[i];

a[i] = a[len-i-1];

a[len-i-1] = temp;

}

*/

return a;

}

char * strrev2(char *p)

{

/*

// string reverse using two pointers

char *s = p;

char *r = p+strlen(p)-1;

char temp;

while(p < r) {

temp = *p;

*p = *r;

*r = temp;

p++; r--;

}

return s;

*/

// how to string reverse using one pointer

int len = strlen(p)-1;

int i;

char temp;

for (i=0; i<len/2; i++)

{

temp = *(p+i);

*(p+i) = *(p+len-i);

*(p+len-i) = temp;

}

return p;

}

/*

extra space of O(n) is required

*/

void reverse(char *s, char *n, int nindex)

{

if (*s == '\0') {

return;

}

reverse (s+1, n, nindex-1);

*(n+nindex) = *s;

return;

}

char * strrev3(char * s)

{

int len = strlen(s);

char *p = (char *) malloc(sizeof(char) * len + 1);

*(p+len) = '\0';

reverse(s, p, len-1);

//free(p);

return p;

}

/*

No extra space, done inplace with two variables

to store the start and the end

*/

void reverse1(char *a, int start, int end)

{

if (start >= end)

return;

reverse1(a, start+1, end-1);

char temp = a[start];

a[start] = a[end];

a[end] = temp;

return;

}

char * strrev4(char *s)

{

reverse1(s, 0, strlen(s)-1);

return s;

}

int main()

{

char s[] = "test string";

char *p = NULL;

p = (char *) malloc(sizeof(char) * strlen("some string") + 1);

strcpy(p, "some string");

printf("strrev1(%s) = ", s);

printf("%s \n", strrev1(s));

printf("strrev1(%s) = ", p);

printf("%s \n", strrev1(p));

free(p);

// ---------------------------------- //

char s1[] = "random dandom";

p = NULL;

p = (char *) malloc(sizeof(char) * strlen("what is this?") + 1);

strcpy(p, "what is this?");

printf("strrev2(%s) = ", s1);

printf("%s \n", strrev2(s1));

printf("strrev2(%s) = ", p);

printf("%s \n", strrev2(p));

free(p);

// ----------------------------------- //

char s2[] = "okie dokie";

p = NULL;

p = (char *) malloc(sizeof(char) * strlen("i don't know!!") + 1);

strcpy(p, "i don't know!!");

printf("strrev3(%s) = ", s2);

printf("%s \n", strrev3(s2));

printf("strrev3(%s) = ", p);

printf("%s \n", strrev3(p));

free(p);

// ----------------------------------- //

char s3[] = "oh! .. come on!!";

p = NULL;

p = (char *) malloc(sizeof(char) * strlen("yet another version of strrev") + 1);

strcpy(p, "yet another version of strrev");

printf("strrev4(%s) = ", s3);

printf("%s \n", strrev4(s3));

printf("strrev4(%s) = ", p);

printf("%s \n", strrev4(p));

free(p);

return 0;

}