Expand Independent Character Choices

### Problem The valid string s describes one or more words. A plain lowercase letter fixes one position; a nonnested brace group such as `{x,y}` offers distinct single-letter choices for that position. Concatenate one choice from each position and return all resulting words in lexicographic order. ```cpp class Solution { public: vector<string> expand(string s); }; ``` ### Examples **Example 1:** ```text Input: s = "q{c,a}{x,z}" Output: ["qax","qaz","qcx","qcz"] Explanation: Choose a or c, then x or z, after the fixed q. ``` **Example 2:** ```text Input: s = "{z,b}a" Output: ["ba","za"] Explanation: Sorting the first-position options determines the word order. ``` ### Constraints - The expression contains 1 to 50 characters. - Only lowercase letters, braces and commas occur, and the input is valid. - Braces never nest. Letters within each brace group are distinct. ### Contract considerations Return the complete sorted list. The ordered result is deterministic. ```hint Read the boundary condition Does a brace group contribute one output character, or should its punctuation appear in the returned word? ``` ### Boundary reminders Return the complete sorted list. The ordered result is deterministic.