1021.cpp

class Solution { // Runtime: 4 ms, faster than 97.26% of C++ online submissions
                 // for Remove Outermost Parentheses.
public:
  string removeOuterParentheses(string S) {
    string res;
    int opened = 0;
    for (char c : S) {
      if (c == '(' && opened++)
        res += c;
      else if (c == ')' && --opened)
        res += c;
    }
    return res;
  }
};