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strrev.c
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strrev.c
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#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;
}