Binary String With Substrings Representing 1 To N LeetCode solution in C++

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class Solution {
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 public:
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  bool queryString(string s, int n) {
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    unordered_set<int> seen;
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    int len = s.length();
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    for (int i = len - 1; i >= 0; i--) {
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      int num = 0;
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      for (int j = 1; j <= min(len - i, 31); j++) { // max 31 bit is enough to form positive integer
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        num = num * 2 + (s[i + j - 1] == '1' ? 1 : 0);
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        if (0 < num && num <= n) seen.insert(num);
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      }
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      if (seen.size() == n) return true;
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    }
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    return false;
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  }
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};