class Solution {public:    vector
    
      letterCasePermutation(string S) {        set
     
       memo;        memo.insert(S);                helper(S, 0, memo);                vector
      
        ans(memo.begin(), memo.end());                return ans;    }    private:    void helper(string S, int idx, set
       
        & memo) {        if (idx >= S.length()) return;        vector
        
          temp(memo.begin(), memo.end());        //cout << idx << endl;        for (string s : temp) {            if (isalpha(s[idx])) {                char c;                if (isupper(s[idx])) c = tolower(s[idx]);                else c = toupper(s[idx]);                //cout << c << endl;                string temp = s;                temp[idx] = c;                if (!memo.count(temp)) memo.insert(temp);                //helper(S, idx+1, memo);            }        }        helper(S, idx+1, memo);    }};