find case perumations
void change_case(char &c){
if(c >= 'A' and c <= 'Z')
c += 32;
else if(c >= 'a' and c <= 'z')
c -= 32;
}
void find_case_permutations(string s, vector<string> &res, size_t index){
res.push_back(s);
for(size_t i = index; i < s.size(); ++i){
string s1 = s;
if(s1[i] >= 'A') change_case(s1[i]);
else continue;
find_case_permutations(s1, res, i+1);
}
}
find_case_permutations("ab7c", res, 0);