who can tell me what the test6 is? Thanks

If you are using sort in your algo and sorting pairs<char ch,int position>, you must compare this pairs by a "ch", but if "ch"`s are equal - you must compare "position". Comparsion function may be like that:

struct s{char c;int p;};

bool cmp(s a, s b)
{
    if(a.c!=b.c)
    {
        return a.c<b.c;
    }
    return a.p<b.p;
}

Use stable_sort instead of sort (if you're doing on C/C++)