Distinct Subsequences

def numDistinct(self, s: str, t: str) -> int:
    l1, l2 = len(s)+1, len(t)+1
    cur = [0] * l2
    cur[0] = 1
    for i in range(1, l1):
        pre = cur[:]
        for j in range(1, l2):
            cur[j] = pre[j] + pre[j-1]*(s[i-1] == t[j-1])
    return cur[-1]