<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:
    std::vector<std::string> removeInvalidParentheses(const std::string& s) {
        std::vector<std::string> result;
        std::unordered_set<std::string> visited;
        std::queue<std::string> queue;
        bool found = false;

        queue.emplace(s);
        visited.emplace(s);

        auto isValid = [](const std::string& s) {
            int balance = 0;
            for (const auto c : s) {
                if (c == '(') {
                    ++balance;
                } else if (c == ')') {
                    --balance;
                    if (balance < 0) {
                        return false;
                    }
                }
            }
            return balance == 0;
        };

        while (!queue.empty()) {
            const auto size = queue.size();
            std::unordered_set<std::string> levelVisited;

            for (std::size_t i = 0; i < size; ++i) {
                const auto curr = queue.front();
                queue.pop();

                if (isValid(curr)) {
                    result.push_back(curr);
                    found = true;
                    continue;
                }

                if (found) {
                    continue;
                }

                for (std::size_t j = 0; j < curr.size(); ++j) {
                    if (curr[j] != '(' && curr[j] != ')') {
                        continue;
                    }

                    if (const auto next =
                            curr.substr(0, j) + curr.substr(j + 1);
                        !visited.contains(next)) {
                        levelVisited.emplace(next);
                    }
                }
            }

            for (const auto& str : levelVisited) {
                queue.emplace(str);
                visited.emplace(str);
            }

            if (found) {
                break;
            }
        }

        return result;
    }
};
51714