Remove Invalid Parentheses

Given a string `s` that contains parentheses and letters, remove the minimum number of invalid parentheses to make the input string valid. Return _a list of **unique strings** that are valid with the minimum number of removals_. You may return the answer in **any order**.   **Example 1:** **Input:** s = "()())()" **Output:** ["(())()","()()()"] ``` **Example 2:** **Input:** s = "(a)())()" **Output:** ["(a())()","(a)()()"] ``` **Example 3:** **Input:** s = ")(" **Output:** [""] ```   **Constraints:** - `1 <= s.length <= 25` - `s` consists of lowercase English letters and parentheses `'('` and `')'`. - There will be at most `20` parentheses in `s`.