Enumerate Every Independent Letter-Case Choice

### Problem For each English letter in s, independently choose uppercase or lowercase; digits stay unchanged. Return every distinct resulting string in any order. ```cpp class Solution { public: vector<string> letterCasePermutation(string s); }; ``` ### Examples **Example 1:** ```text s = "B7" One valid output: ["b7","B7"] Explanation: The one letter has two choices. ``` **Example 2:** ```text s = "42" Output: ["42"] Explanation: Digits offer no case choice. ``` ### Constraints - String length ranges from 1 through 12. - Characters are English letters of either case or decimal digits. ### Contract considerations Preserve digits and the full list of all case assignments. ```hint Read the boundary condition How many independent choices does a letter introduce? ``` ### What a Strong Answer Covers Preserve digits and the full list of all case assignments. ### Follow-up Questions Optional practice extension: How could results be yielded one at a time when storing the whole list is undesirable?