Count Number of Special Subsequences LeetCode solution in C++

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class Solution {
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 public:
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  int countSpecialSubsequences(vector<int> &nums) {
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    long long z = 0, o = 0, t = 0;
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    long long mod = 1e9 + 7;
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    for (int i = 0; i < nums.size(); i++) {
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      if (nums[i] == 0)
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        z = (z + z + 1) % mod;
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      else if (nums[i] == 1)
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        o = (o + o + z) % mod;
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      else
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        t = (t + t + o) % mod;
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    }
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    return (((t % mod) + mod) % mod);
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  }
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};