Count Substrings with Only One Distinct Letter

lcpy gen -n 1180   # by problem number
lcpy gen -s count_substrings_with_only_one_distinct_letter   # by problem name

Input: s = "aaaba"
Output: 8
Explanation: The substrings with one distinct letter are "aaa", "aa", "a", "b".
"aaa" occurs 1 time.
"aa" occurs 2 times.
"a" occurs 4 times.
"b" occurs 1 time.
So the answer is 1 + 2 + 4 + 1 = 8.

Input: s = "aaaaaaaaaa"
Output: 55

class Solution:
    # Time: O(n)
    # Space: O(1)
    def count_letters(self, s: str) -> int:
        ans = 0
        i = 0
        n = len(s)
        while i < n:
            j = i
            while j < n and s[j] == s[i]:
                ans += j - i + 1
                j += 1
            i = j
        return ans