921. Minimum Add to Make Parentheses Valid

A parentheses string is valid if and only if:

• It is the empty string,
• 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.

You are given a parentheses string s. In one move, you can insert a parenthesis at any position of the string.

• For example, if s = “()))”, you can insert an opening parenthesis to be “(()))” or a closing parenthesis to be “())))”.

Return the minimum number of moves required to make s valid.

class Solution {
public:
    int minAddToMakeValid(string s) {
        vector<char> st;
        for(auto& c : s) {
            if(st.empty() || c == '(') st.push_back(c);
            else if(st.back() == '(') st.pop_back();
            else st.push_back(c);
        }
        
        return st.size();
    }
};