Number of Substrings Containing All Three

lcpy gen -n 1358   # by problem number
lcpy gen -s number_of_substrings_containing_all_three_characters   # by problem name

Input: s = "abcabc"
Output: 10
Explanation: The substrings containing at least one occurrence of the characters a, b and c are "abc", "abca", "abcab", "abcabc", "bca", "bcab", "bcabc", "cab", "cabc" and "abc" (again).

Input: s = "aaacb"
Output: 3
Explanation: The substrings containing at least one occurrence of the characters a, b and c are "aaacb", "aacb" and "acb".

Input: s = "abc"
Output: 1

class Solution:
    def number_of_substrings(self, s: str) -> int:
        count = 0
        last = [-1, -1, -1]
        for i, ch in enumerate(s):
            last[ord(ch) - 97] = i
            count += min(last) + 1
        return count