115. Distinct Subsequences

https://leetcode.com/problems/distinct-subsequences/

dp[i][j] 存的是当s[:i] 包含多少个 t[:j] 需要注意的是dp[i][0] 需要初始化，当t是empty string时，count是1

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