115不同的子序列

# 看着大佬的题解做了出来，但写不出来题解

class Solution:
    def numDistinct(self,s:str,t:str) -> int:
        n1 = len(s)
        n2 = len(t)
        dp = [[0] * (n1 + 1) for _ in range((n2 + 1))]
        for j in range(n1 + 1):
            dp[0][j] = 1
        for i in range(1,n2 + 1):
            for j in range(1,n1 + 1):
                if t[i - 1] == s[j - 1]:
                    dp[i][j] = dp[i-1][j - 1] + dp[i][j-1]
                else:
                    dp[i][j] = dp[i][j - 1]
        return dp[-1][-1]