Smallest Subsequence Keeping Each Letter Once

### Problem Return the lexicographically smallest subsequence of s that includes each distinct letter appearing in s exactly once. Character order must follow the input, and no distinct letter may be omitted. ```cpp class Solution { public: string smallestSubsequence(string s); }; ``` ### Examples **Example 1:** ```text Input: s = "bbaa" Output: "ba" Explanation: Both letters are required, but every b precedes every a, so ab is not a subsequence. ``` **Example 2:** ```text Input: s = "dcabdc" Output: "abdc" Explanation: The suffix abdc includes each of a,b,c,d once and begins with the smallest feasible first letter. ``` ### Constraints - The input has 1 to 1000 letters. - Only lowercase English letters occur. ### Contract considerations All distinct letters must appear, and lexicographic minimization makes the answer deterministic. ```hint Read the boundary condition Which requirements must hold together: input order, every distinct letter, and exactly one copy of each? ``` ### Boundary reminders All distinct letters must appear, and lexicographic minimization makes the answer deterministic.