784 Letter Case Permutation

class Solution { public: void permutationHelper(int pos, string S, string chosen, vector<string> & res){ if (chosen.size()==S.size()){ res.push_back(chosen); return; } char c = S[pos]; if (isalpha(c)){ chosen.push_back(tolower(c)); permutationHelper(pos+1, S, chosen, res); chosen.pop_back(); chosen.push_back(toupper(c)); permutationHelper(pos+1, S, chosen, res); chosen.pop_back(); } else { chosen.push_back(c); permutationHelper(pos+1, S, chosen, res); chosen.pop_back(); } } vector<string> letterCasePermutation(string S) { vector<string> res; permutationHelper(0, S, "", res); return res; } };