Next Greater Element III LeetCode solution in C++

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class Solution {
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 public:
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  int nextGreaterElement(int n) {
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    vector<int> vec;
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    int temp = n;
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    while (n > 0) {
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      int r = n % 10;
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      vec.push_back(r);
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      n /= 10;
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    }
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    sort(vec.begin(), vec.end());
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    do {
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      int num = 0;
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      long j = 0;
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      int s = vec.size() - 1;
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      long i = pow(10, s);
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      while (i > 0) {
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        num += i * vec[j++];
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        i /= 10;
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      }
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      if (num > temp) return num;
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    } while (next_permutation(vec.begin(), vec.end()));
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    return -1;
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  }
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};