Count Substrings with Only One Distinct Letter

Given a string s, return the number of substrings that have only one distinct letter.

Example 1:

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.

Example 2:

Input: s = "aaaaaaaaaa"

Output: 55

Constraints:

• 1 <= s.length <= 1000
• s[i] consists of only lowercase English letters.