public int numDistinct(String S, String T) {

int M = S.length();

int N = T.length();

int[] dp = new int[N+1];

Arrays.fill(dp, 0);

dp[0] = 1;

for (int i = 1; i <= M; ++i) {

for (int j = N; j >=1; --j) {

dp[j] = dp[j] + (S.charAt(i-1) == T.charAt(j-1) ? dp[j-1] : 0);

}

}

return dp[N];

}

}