<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]:
        self.res = []
        self.backtracking(s, 0, len(s), "")
        max_len = 0
        for valid in self.res:
            max_len = max(max_len, len(valid))
        
        res = set()
        for valid in self.res:
            if len(valid) == max_len:
                res.add(valid) 

        return res
    
    def backtracking(self, s, idx, length, track):
        if idx == length:
            if self.isValid(track):
                self.res.append(track)
            return
        
        if s[idx] != '(' and s[idx] != ')':
            track += s[idx]
            self.backtracking(s, idx + 1, length, track)
        
        else:
            self.backtracking(s, idx + 1, length, track)
            track += s[idx]
            self.backtracking(s, idx + 1, length, track)
        
        return


    def isValid(self, s):

        left = 0
        for c in s:
            if c == '(':
                left += 1
            elif c == ')':
                left -= 1
            
            if left < 0:
                return False
        
        if left != 0:
            return False
        else:
            return True
    
        
18500