Minimum Remove to Make Valid Parentheses

Given a string s of '(' , ')' and lowercase English characters.

Your task is to remove the minimum number of parentheses ( '(' or ')', in any positions ) so that the resulting parentheses string is valid and return any valid string.

Formally, a parentheses string is valid if and only if:

• It is the empty string, contains only lowercase characters, or
• It can be written as AB (A concatenated with B), where A and B are valid strings, or
• It can be written as (A), where A is a valid string.

Example 1:

Input: s = "lee(t(c)o)de)"
Output: "lee(t(c)o)de"
Explanation: "lee(t(co)de)" , "lee(t(c)ode)" would also be accepted.

Example 2:

Input: s = "a)b(c)d"
Output: "ab(c)d"

Example 3:

Input: s = "))(("
Output: ""
Explanation: An empty string is also valid.

Constraints:

• 1 <= s.length <= 105
• s[i] is either'(' , ')', or lowercase English letter.

SOLUTION:

class Solution:
    def minRemoveToMakeValid(self, s: str) -> str:
        op = set()
        n = len(s)
        stack = []
        for i in range(n):
            if s[i] == '(':
                stack.append(i)
                op.add(i)
            elif s[i] == ')':
                if len(stack) > 0:
                    stack.pop()
                    op.add(i)
            else:
                op.add(i)
        for i in stack:
            if i in op:
                op.remove(i)
        return "".join(s[i] for i in sorted(op))