【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); } } };