【easy】784. Letter Case Permutation

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]

class Solution {
public:
    vector<string> letterCasePermutation(string S) {
        vector<string> res;
        int idx = 0;
        helper(S, idx, res);
        return res;
    }
    
    void helper(string& s, int idx, vector<string>& res) {
        if (idx == s.size()) {
            res.push_back(s);
            return;
        }
        helper(s, idx + 1, res);
        if (isalpha(s[idx])) {
            s[idx] = islower(s[idx]) ? toupper(s[idx]) : tolower(s[idx]);
            helper(s, idx + 1, res);
        }
    }
};