[LeetCode] Distinct Subsequence

Given a string S and a string T, count the number of distinct subsequences of T in S.
A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, "ACE" is a subsequence of "ABCDE" while "AEC" is not).
Here is an example:
S = "rabbbit", T = "rabbit"
Return 3.

解题思路：
      DP思想 ：1.  S(i) -> T(j) = S(i-1)->T(j)  if s.charAt(i) !=  t.charAt(j)
                        2. S(i) -> T(j) = S(i-1) -> T(j-1) + S(i-1)->T(j) if s.charAt(i) ==  t.charAt(j)

public int numDistinct(String S, String T) {
       int row = S.length();
       int col = T.length();
       int[][] num = new int[row+1][col+1];
       
       for(int i = 0; i <= row; i++){
           for(int j = 0; j <= col; j++){
               if(i==0){
                   if(j == 0) num[i][j] = 1;
                   else num[i][j] = 0;
               }else if(j == 0){
                   num[i][j] = 1;
               }else{
                   num[i][j] = num[i-1][j];
                   
                   if(S.charAt(i-1) == T.charAt(j-1)){
                       num[i][j] +=num[i-1][j-1];
                   }
               } 
           }
       }
       
       return num[row][col];
       
    }