knr/4.13.c

#include  <stdio.h>
void rvrs(char[]);
int rev (char [], int , int );
void swap(char s[], int in, int out);

int main()
{
  char x[]="1 2 3 4 5 6 7 8 9 10 11 12";
  rvrs(x);
  printf ("%s\n",x);
  return 0;   
}

/** recursively reverse string, lisp style */
  //	 (defun rev (s)

void rvrs(char in[])
{
  if (in!=0)
    rev(in,0,0);
}

int rev (char in[], int inpos, int outpos) //returns outpos
{
  //	  (if (> (length s) 1)
  //  if (in!=0)  we check this above
    if (in[inpos]!='\0')
	 {    	   
	   if(in[inpos+1]!='\0')
		//	  (append (rev (cdr s)) (list (car s)))
		{
		  int outposnew=rev(in,inpos+1,outpos); // O(1)*O(N) ~ O(N) space reverse O(N) time.
			 if (inpos > outposnew)
			   {
				swap(in,inpos,outposnew);
				return outposnew+1;
			   }
		    	  return outposnew;
		}
	   else { // last element
		//	  s)
		if (inpos > outpos)
		  {
		    swap(in,inpos,outpos);
		    return outpos+1;
		  }
		return outpos;
	   }
	 }
}

void swap(char s[], int in, int out)
{
		  // swap
		  char temp;
		  temp=s[in];
		  s[in]=s[out];
		  s[out]=temp;
}