Bitwise ORs of Subarrays LeetCode solution in C++

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class Solution {
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 public:
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  int subarrayBitwiseORs(vector<int> &arr) {
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    vector<int> s;
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    int l = 0;
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    for (int a : arr) {
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      int r = s.size();
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      s.push_back(a);
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      for (int i = l; i < r; i++)
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        if (s.back() != (s[i] | a)) s.push_back(s[i] | a);
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      l = r;
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    }
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    return unordered_set<int>(begin(s), end(s)).size();
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  }
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};