代码随想录：不同的子序列

说实话有点没搞懂

class Solution {
public:
    int numDistinct(string s, string t) {
        int m = s.length();
        int n = t.length();
        vector<vector<unsigned long long>> dp =
            vector<vector<unsigned long long>>(m + 1, vector<unsigned long long>(n + 1, 0));
        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++) {
                if (s[i - 1] == t[j - 1])
                //转移方程代表，给大字符串新增一位，如果匹配上了，就是不增加的时候的结果+新增的结果
                    dp[i][j] = dp[i - 1][j - 1] + dp[i - 1][j];
                else
                    dp[i][j] = dp[i - 1][j];
            }
        }
        return dp[m][n];
    }
};