ValidString

import java.util.Stack;
 
public class ValidString {
    public String minRemovalToMakeValid(String s) {
        Stack<Integer> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c == '(') {
                stack.push(i);
            } else if (c == ')') {
                if (!stack.isEmpty() && s.charAt(stack.peek()) == '(') {
                    stack.pop();
                } else {
                    stack.push(i);
                }
            }
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            if (!stack.contains(i)) {
                sb.append(s.charAt(i));
            }
        }
        return sb.toString();
    }
}