```
int numDistinct(string s, string t) {
int len1 = s.length();
int len2 = t.length();
if(len1 == 0 || len2 == 0){
return 0;
}
vector<vector<int>> dp(len1 + 1, vector<int>(len2 + 1, 0));
dp[0][0] = 1;
for(int i = 1; i <= len1; ++i){
dp[i][0] = 1;
for(int j = 1; j <= i && j <= len2; ++j){
dp[i][j] = dp[i - 1][j];
if(s[i - 1] == t[j - 1]){
dp[i][j] += dp[i - 1][j - 1];
}
}
}
return dp[len1][len2];
}
```