1190. Reverse Substrings Between Each Pair of Parentheses

class Solution {
public:
    void reverseString(string &s, int i, int j)
    {
        while(i < j)
        {
            if(s[i] == '(') i++;
            if(s[j] == ')') j--;
            swap(s[i++], s[j--]);
        }
    }
    string reverseParentheses(string s) 
    {
        stack<int>st;
        int n = s.size();
        for(int i = 0; i < n; i++)
        {
            if(s[i] == '(') st.push(i);
            if(s[i] == ')')
            {
                reverseString(s, st.top(), i);
                st.pop();
            }
        }

        string ans;
        for(auto ch:s)
            if(isalpha(ch)) ans += ch;
        return ans;
    }
};