Distinct Subsequences

public class Solution {
public int numDistinct(String s, String t) {
if (s == null || t == null) {
return 0;
}
int[][] sums = new int[s.length() + 1][t.length() + 1];
//sums[0][0] = 1;
for (int i = 0; i < s.length(); i++) {
sums[i][0] = 1;
}
for (int i = 1; i <= s.length(); i++) {
for (int j = 1; j <= t.length(); j++) {
sums[i][j] = sums[i - 1][j];
if (s.charAt(i - 1) == t.charAt(j - 1)) {
sums[i][j] += sums[i - 1][j - 1];
}
}
}
return sums[s.length()][t.length()];
}
}