1249. Minimum Remove to Make Valid Parentheses

T: O(n)
S: O(n)

```java
class Solution {
    public String minRemoveToMakeValid(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.push(i);
                } else {
                    stack.pop();
                }
            }
        }
        StringBuilder sb = new StringBuilder(s);
        while (!stack.isEmpty()) {
            sb.deleteCharAt(stack.pop());
        }
        return sb.toString();
    }
}
```