Opgaveløsning:
Funktionen strrev

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

char *strrev(char *str);

int main(void) {

  char str[] = "abcd";
  printf("%s\n", strrev(str));

  strcpy(str, "abc");
  printf("%s\n", strrev(str));

  strcpy(str, "ab");
  printf("%s\n", strrev(str));

  strcpy(str, "a");
  printf("%s\n", strrev(str));

  strcpy(str, "");
  printf("%s\n", strrev(str));
  
  return 0;
}

char *strrev(char *str){
  int i = 0,
      j = strlen(str) - 1;
  char temp;

  while (i < j){
   
    // Swap char number i and j in str:
    temp = str[j];
    str[j] = str[i];
    str[i] = temp;

    i++; j--;
  }

  return str;
}

dcba
cba
ba
a