<p>Given a string <code>s</code> that contains parentheses and letters, remove the minimum number of invalid parentheses to make the input string valid.</p>

<p>Return <em>a list of <strong>unique strings</strong> that are valid with the minimum number of removals</em>. You may return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;()())()&quot;
<strong>Output:</strong> [&quot;(())()&quot;,&quot;()()()&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;(a)())()&quot;
<strong>Output:</strong> [&quot;(a())()&quot;,&quot;(a)()()&quot;]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;)(&quot;
<strong>Output:</strong> [&quot;&quot;]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 25</code></li>
	<li><code>s</code> consists of lowercase English letters and parentheses <code>&#39;(&#39;</code> and <code>&#39;)&#39;</code>.</li>
	<li>There will be at most <code>20</code> parentheses in <code>s</code>.</li>
</ul>

class Solution:
    def removeInvalidParentheses(self, s: str) -> List[str]: 
        result = []
        def isValid(string):
            stack = []
            for ch in string:
                if ch == "(":
                    stack.append(ch)
                elif ch == ")":
                    if not stack:
                        return False
                    stack.pop()
            return len(stack) == 0
        
        # try all the options
        def helper(stringSoFar, remaining):
            if not remaining:
                if isValid(stringSoFar):
                    result.append(stringSoFar)
            else:
                # if character, just append it
                if remaining[0] not in ")(":
                    stringSoFar += remaining[0]
                    remaining = remaining[1:]
                    helper(stringSoFar, remaining)
                else:
                    helper(stringSoFar, remaining[1:])
                    helper(stringSoFar+remaining[0], remaining[1:])    
        helper("", s)
        # find the length of the max string
        maxLength = max([len(a) for a in result])
        result = [a for a in result if len(a) == maxLength]
        return set(result)
19100