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.
Example 4:
Input: s = "(a(b(c)d)"
Output: "a(b(c)d)"
Constraints:
1 <= s.length <= 10^5
s[i] is one of  '(' , ')' and lowercase English letters.
class Solution:
    def minRemoveToMakeValid(self, s: str) -> str:
        par_stack = []
        for i, ch in enumerate(s):
            if ch == '(' or ch == ')':
                par_stack.append((i, ch))
                while len(par_stack) >= 2 and par_stack[-1][1] == ')' and par_stack[-2][1] == '(':
                    par_stack.pop()
                    par_stack.pop()
        
        invalid = {x[0]: True for x in par_stack}
        ret = [ch for i, ch in enumerate(s) if i not in invalid]
        return ''.join(ret)
class Solution:
    def minRemoveToMakeValid(self, s: str) -> str:
        par_stack = []
        for i, ch in enumerate(s):
            if ch == '(' or ch == ')':
                par_stack.append((i, ch))
                while len(par_stack) >= 2 and par_stack[-1][1] == ')' and par_stack[-2][1] == '(':
                    par_stack.pop()
                    par_stack.pop()
        
        invalid = {x[0]: True for x in par_stack}
        ret = [ch for i, ch in enumerate(s) if i not in invalid]
        return ''.join(ret)