#include <string.h>
 
int main()
{
    char str[10] = "abcdef";
    int slen = strlen(str);
 
    for(int i=0;i<slen/2;i++)
    {
        str[i] = str[i] + str[slen-i-1];
        str[slen-i-1] = str[i] - str[slen-i-1];
        str[i] = str[i] - str[slen-i-1];
    }
 
    return 0;
}