<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 {
public:
 
    vector<string>ans;
    unordered_map<string,int>mp;
    int getvalid(string s){
        stack<char>st;
        int i=0;
        while(i<s.size()){
            if(s[i] == '(')st.push('(');
            else if(s[i] == ')'){
                if(st.size() && st.top() == '(')st.pop();
                else  st.push(')');
            }
            i++;
        }
        return st.size();
    }
    void solve(string s,int minv){
        if(mp[s] != 0)return;
        else mp[s]++;
        if(minv <0)return;
        if(minv == 0){
            if(!getvalid(s)){
                ans.push_back(s);
                return;
            }
        }
        for(int i=0;i<s.size();i++){
              string l = s.substr(0,i);
              string r = s.substr(i+1);
              solve(l+r,minv-1);
        }
        return;

    }
    vector<string> removeInvalidParentheses(string s) {
        solve(s,getvalid(s));
        return ans;
    }
};
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