distinct_seq.cc

class Solution {
public:
    int numDistinct(string S, string T) {
        return numDistinct(S, T, 0, 0);
    }
    
    int numDistinct(string S, string T, int s, int t) {
        if (t == T.size()) {
            return 1;
        }
        if (s == S.size()) {
            return 0;
        }
        if (S[s] == T[t]) {
            return numDistinct(S, T, s + 1, t + 1) + numDistinct(S, T, s + 1, t);
        } else {
            return numDistinct(S, T, s + 1, t);
        }
    }
};