<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 {
private:
    bool isValid(string s) {
        stack<char> st;

        for(char c: s) {
            if(c == '(') {
                st.push(c);
            }
            else if (c == ')') {
                if(st.empty())
                    return false;
                st.pop();
            }
        }
        
        return st.empty();
    }

public:
    vector<string> removeInvalidParentheses(string s) {
        vector<string> ans;
        queue<string> q;
        q.push(s);
        unordered_map <string, bool> mp;

        while(q.size()) {
            string top = q.front();
            q.pop();
            
            if(mp[top])
                continue;

            mp[top] = true;
            
            if(isValid(top)) 
                ans.push_back(top);


            else if(ans.empty()) {
                for(int i=0; i<top.size(); ++i) {
                    if(top[i] == '(' or top[i] == ')') {
                        q.push(top.substr(0, i) + top.substr(i+1));
                    }
                }
            }
        }

        return ans;
    }
};
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