Substrings of Size Three with Distinct Characters

A string is good if there are no repeated characters.

Given a string s​​​​​, return the number of good substrings of length three in s​​​​​​.

Note that if there are multiple occurrences of the same substring, every occurrence should be counted.

A substring is a contiguous sequence of characters in a string.

Example 1:

Input: s = "xyzzaz"
Output: 1
Explanation: There are 4 substrings of size 3: "xyz", "yzz", "zza", and "zaz".
The only good substring of length 3 is "xyz".

Example 2:

Input: s = "aababcabc"
Output: 4
Explanation: There are 7 substrings of size 3: "aab", "aba", "bab", "abc", "bca", "cab", and "abc".
The good substrings are "abc", "bca", "cab", and "abc".

Constraints:

• 1 <= s.length <= 100
• s​​​​​​ consists of lowercase English letters.

SOLUTION:

class Solution:
    def countGoodSubstrings(self, s: str) -> int:
        n = len(s)
        if n < 3:
            return 0
        dist = 0
        ctr = [0] * 26
        for i in range(3):
            ctr[ord(s[i]) - ord('a')] += 1
        for i in range(n - 3):
            if max(ctr) == 1:
                dist += 1
            ctr[ord(s[i]) - ord('a')] -= 1
            ctr[ord(s[i+3]) - ord('a')] += 1
        if set(ctr) == {0, 1}:
            dist += 1
        return dist