Numbers With Same Consecutive Differences LeetCode solution in C++

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class Solution {
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 public:
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  vector<int> res;
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  void solve(int num, int n, int k) {
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    if (n == 1) {
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      res.push_back(num);
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      return;
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    }
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    int dig = num % 10;
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    if (dig + k <= 9 && k != 0) {
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      solve(num * 10 + dig + k, n - 1, k);
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    }
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    if (dig - k >= 0) {
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      solve(num * 10 + dig - k, n - 1, k);
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    }
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  }
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  vector<int> numsSameConsecDiff(int n, int k) {
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    res.clear();
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    for (int i = 1; i <= 9; ++i) {
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      solve(i, n, k);
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    }
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    return res;
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  }
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};