Number of Ways to Select Buildings LeetCode solution in C++

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class Solution {
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 public:
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  long long numberOfWays(string s) {
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    long long a = 0, b = 0,
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              ans = 0; // a and b are the number of occurances of '1' and '0'
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                       // after the current building respectively.
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    for (int i = 0; i < s.length(); i++) {
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      if (s[i] == '1')
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        a++;
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      else
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        b++;
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    }
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    long long c = 0, d = 0; // c and d are the number of occurances of '1' and
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                            // '0' before the current building respectively.
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    for (int i = 0; i < s.length(); i++) {
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      if (s[i] == '1') { // Counting the sequences of "010"
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        ans += (d * b);
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        a--;
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        c++;
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      } else { // Counting the sequences of "101"
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        ans += (a * c);
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        b--;
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        d++;
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      }
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    }
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    return ans;
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  }
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};