Leetcode 115: Distinct Subsequences

Given a string S and a string T, count the number of distinct subsequences of S which equals T.

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.

 

public class Solution {
    public int NumDistinct(string s, string t) {
        int sLen = s.Length, tLen = t.Length;
        
        if (sLen < tLen) return 0;        
        if (tLen == 0) return 1;
        
        var dp = new int[tLen + 1, sLen + 1];
        for (int j = 0; j <= sLen; j++)
        {
            dp[0, j] = 1;
        }
        
        for (int j = 1; j <= tLen; j++)
        {
            dp[j, 0] = 0;
        }
        
        for (int i = 1; i <= tLen; i++)
        {
            for (int j = i; j <= sLen; j++)
            {
                if (s[j - 1] == t[i - 1])
                {
                    dp[i, j] =  dp[i - 1, j - 1] + dp[i, j - 1];
                }
                else
                {
                    dp[i, j] = dp[i, j - 1];
                }
            }
        }
        
        return dp[tLen, sLen];
    }
}