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.

Solution

Time complexity : O(n)
Space complexity : O(1)

class Solution {
public:
    string minRemoveToMakeValid(string s) {
        int close = count(s.begin(), s.end(), ')');
        int open = 0;
        
        string ans;
        for (char c: s) {
            if (c == '(') {
                if (close <= open)
                    continue;
                ++open;
            } else if (c ==')') {
                --close;
                if (open <= 0)
                    continue;
                --open;
            }
            ans += c;
        }
        return ans;
    }
};