Distinct Subsequences II Python Solution

lcpy gen -n 940   # by problem number
lcpy gen -s distinct_subsequences_ii   # by problem name

Input: s = "abc"
Output: 7
Explanation: The 7 distinct subsequences are "a", "b", "c", "ab", "ac", "bc", and "abc".

Input: s = "aba"
Output: 6
Explanation: The 6 distinct subsequences are "a", "b", "ab", "aa", "ba", and "aba".

Input: s = "aaa"
Output: 3
Explanation: The 3 distinct subsequences are "a", "aa" and "aaa".

class Solution:
    # Time: O(n)
    # Space: O(1)

    def distinct_subseq_ii(self, s: str) -> int:
        mod = 10**9 + 7
        end: list[int] = [0] * 26
        total = 1
        for ch in s:
            c = ord(ch) - 97
            end[c], total = total, (2 * total - end[c]) % mod
        return (total - 1) % mod