1. class Solution:
    2.     def numDistinct(self, s: str, t: str) -> int:
    3.         n = len(s)
    4.         m = len(t)
    5.         if n < m:
    6.             return 0
    8.         dp = [[0] * (m + 1) for _ in range(n + 1)]
    9.         dp[0][0] = 1
    10.         for i in range(1, n+1):
    11.             for j in range(m+1):
    12.                 if j == 0:
    13.                     dp[i][j] = 1
    14.                 elif s[i-1] == t[j-1]:
    15.                     dp[i][j] = dp[i-1][j-1] + dp[i-1][j]
    16.                 else:
    17.                     dp[i][j] = dp[i-1][j]
    19.         return dp[-1][-1]