Generate Parentheses LeetCode solution in C++

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class Solution {
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 private:
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  void solve(int cnt1, int cnt2, int n, vector<string> &ans, string &op, int ind) {
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    if (ind >= n * 2) {
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      ans.push_back(op);
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      return;
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    }
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    if (cnt1 < n and ind < n * 2 - 1) {
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      op += '(';
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      solve(cnt1 + 1, cnt2, n, ans, op, ind + 1);
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      op.pop_back();
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    }
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    if (cnt2 < cnt1 and ind > 0) {
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      op += ')';
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      solve(cnt1, cnt2 + 1, n, ans, op, ind + 1);
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      op.pop_back();
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    }
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  }
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 public:
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  vector<string> generateParenthesis(int n) {
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    string op = "";
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    vector<string> ans;
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    solve(0, 0, n, ans, op, 0);
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    return ans;
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  }
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};