115. distinct subsequences

class Solution:
    def numDistinct(self, s, t):
        def find_all(a, b, i):
            while i < len(a):
                j = a.find(b, i)
                yield j
                if j == -1:
                    break
                i = j + 1

        def helper(i, j):
            res = 0
            print(i, j)
            if j == len(t):
                return 1
            for k in find_all(s, t[j], i):
                if k != -1:
                    res += helper(k+1, j+1)
            return res

        return helper(0, 0)

class Solution:
    def numDistinct(self, s, t):
        if len(t) == 0:
            return 1
        if len(s) == 0:
            return 0
        if s[-1] == t[-1]:
            return self.numDistinct(s[:-1], t[:-1]) + self.numDistinct(s[:-1], t)
        else:
            return self.numDistinct(s[:-1], t)