Antonio Gulli's coding playground

no additional space:

void reverse(char str[]){
int i,size = strlen(str);
for (i = 0; i < size/2; i++){
str[i] = str[i] ^ str[size - i - 1];
str[size - i - 1] = str[i] ^ str[size - i - 1];
str[i] = str[i] ^ str[size - i - 1];
}
return;
}