1249. Minimum Remove to Make Valid Parent

set string

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.

分析：用栈得到需要去掉的所有index，然后从s里去除掉这些Index

from collections import deque
class Solution:
    def minRemoveToMakeValid(self, s: str) -> str:
        stack = []
        to_remove = set()
        for i,c in enumerate(s):
            if c == '(':
                stack.append(i)
            elif c == ')':
                if stack:
                    stack.pop()
                else:
                    to_remove.add(i)
        to_remove.update(stack)

        return ''.join(c for i,c in enumerate(s) if i not in to_remove)