Reverse a string

Question

Write code to reverse a string in its place.

Answer

[ This basically means, that we cannot allocate extra memory for the reversed string and have to return it back in the same memory ]

To reverse a string in its place, we'll have to swap the 1^st character with the n^th character, then the 2^nd character with the (n - 1)^th character and so on ...

void reverse_str (char **s)
{
   char *t = *s;
   char *left = t;
   /* TODO: make sure that t is not NULL */
   char *right = t + strlen (t) - 1; 
   char tmp;

   while (left -lt right)
   {
      tmp = *left;
      *left = *right;
      *right = tmp;
      left ++;
      right --;
   }
}

Corollary Question

Reverse the string in its place, without using a temporary variable.

Answer

The logic for this comes from the simple swapping technique that does not use a temporary variable.

swap ( int *a, int *b)
{
   /* Lets start with a = 10 and b = 50

   *a = *a + *b;  /* a = 60, b = 50 */
   *b = *a - *b;  /* a = 60, b = 10 */
   *a = *a - *b;  /* a = 50, b = 10 */

}

Using the above logic we can re-write our function to reverse the string as below

void reverse_str (char **s)
{
   char *t = *s;
   char *left = t;
   /* TODO: make sure that t is not NULL */
   char *right = t + strlen (t) - 1; 

   while (left -lt right)
   {
      *left  = *left + *right;
      *right = *left - *right;
      *left  = *left - *right;
      left ++;
      right --;
   }
}