Distinct Subsequences

<-H 115> Distinct Subsequences

class Solution {
public:
    int numDistinct(string s, string t) {
        vector<unsigned long long> f(t.size() + 1);

        f[0] = 1;
            for(int i = 0; i < s.size(); i++)
                for(int j = t.size() - 1; j >= 0; j--)
                    f[j + 1] += s[i] == t[j] ? f[j] : 0;

        return f[t.size()];
    }
};