# 301 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 all the possible results. 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`.
 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 `````` ``````class Solution: def removeInvalidParentheses(self, s: str) -> List[str]: def is_valid(string: str) -> bool: count = 0 for c in string: if c == '(': count += 1 elif c == ')': count -= 1 if count < 0: return False return count == 0 level = {s} while True: valid = list(filter(is_valid, level)) if valid: return valid level = { w[:i] + w[i+1:] for w in level for i in range(len(w)) if w[i] in {'(', ')'} } return ['']``````