reverse2.c

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

void swap(char *c1, char *c2){
	*c1 ^= *c2 ^= *c1 ^= *c2;
}


char *reverse_iterative(char *s){
	int len = strlen(s);
	int i;
	for ( i = 0; i < len / 2; i++){
		swap(s+i, s+(len-1-i));
	}
	return s;
}




char *rev(char *s){
	static char *tail = NULL;
	static char *head = NULL;

	if (tail == NULL){
		head = tail = s;
	// move tail to end of string.
		while(*tail++);
		tail -= 2;
	}
	if (head - tail >= 0){
		return s;
	}
	// Maybe we can detect palindrome.
	swap(head++, tail--);
	return rev(s);
}


int main(){
	char str[] = "hello";
	printf("%s\n", 	rev(str));
	return 0;
}