All Possible Combinations

• Time : $O(|A_i|log(|A_i|)mul(|A_i|))$
• Space : $O(mul(|A_i|))$

void helper(vector<string>& A, vector<string>& res, int p, string& now) {
    if(p == A.size()) res.push_back(now);
    else {
        for(auto& ch : A[p]) {
            now.push_back(ch);
            helper(A,res,p+1,now);
            now.pop_back();
        }
    }
}

vector<string> Solution::specialStrings(vector<string> &A) {
    vector<string> res;
    string now = "";
    for(auto& a : A) sort(begin(a), end(a));
    helper(A,res,0, now);
    return res;
}