Given a string s, return the total number of substrings that contain only one distinct letter.
s
Constraints:
1≤1 \leq1≤ s.length ≤1000\leq 1000≤1000
s.length
s[i] consists of only lowercase English letters.
s[i]
The key insight is that every position in the string ends some number of single-letter substrings, and that ...