DistinctSubsequences.java

public class Solution {
    public int numDistinct(String S, String T) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int m = T.length(), n = S.length();
        int[] f = new int[m + 1];
        f[0] = 1;
        for (int i = 1; i <= n; ++i) {
            for (int j = m; j >= 1; --j) {
                f[j] += (T.charAt(j - 1) == S.charAt(i - 1)) 
                        ? f[j - 1] : 0;
            }
        }
        return f[m];
    }
}

public class Solution {
    public int numDistinct(String s, String t) {
        int m = s.length(), n = t.length();
        int[][] dp = new int[m + 1][n + 1];
        for (int i = 0; i <= m; ++i) {
            dp[i][0] = 1;
        }
        for (int i = 1; i <= m; ++i) {
            for (int j = 1; j <= n; ++j) {
                dp[i][j] = dp[i - 1][j] + (s.charAt(i - 1) == t.charAt(j - 1) ? dp[i - 1][j - 1] : 0);
            }
        }
        return dp[m][n];
    }
}