LeetCode: Distinct Subsequences

一开始dfs没过large， 网上看了别人的dp，哎，这题比较难。。

class Solution {
public:
    int numDistinct(string S, string T) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int>> f(S.size()+1, vector<int>(T.size()+1));
        for (int i = 0; i <= S.size(); i++) f[i][0] = 0;
        for (int i = 0; i <= T.size(); i++) f[0][i] = 0;
        for (int i = 1; i <= S.size(); i++) {
            if (S[i-1] == T[0]) f[i][1] = f[i-1][1]+1;
            else f[i][1] = f[i-1][1];
        }
        for (int i = 2; i <= S.size(); i++) {
            for (int j = 2; j <= T.size(); j++) {
                if (S[i-1] == T[j-1]) f[i][j] = f[i-1][j-1]+f[i-1][j];
                else f[i][j] = f[i-1][j];
            }
        }
        return f[S.size()][T.size()];
    }
};

 c#

public class Solution {
    public int NumDistinct(string s, string t) {
        int[,] f = new int[s.Length+1, t.Length+1];
        for (int i = 1; i <= s.Length; i++) {
            if (s[i-1] == t[0]) f[i, 1] = f[i-1, 1] + 1;
            else f[i, 1] = f[i-1, 1];
        }
        for (int i = 2; i <= s.Length; i++) {
            for (int j = 2; j <= t.Length; j++) {
                if (s[i-1] == t[j-1]) f[i, j] = f[i-1, j-1] + f[i-1, j];
                else f[i, j] = f[i-1, j];
            }
        }
        return f[s.Length, t.Length];
    }
}