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Substrings that begin and end with the same letter
You are given a 0-indexed string s consisting of only lowercase English letters. Return the number of substrings in s that begin and end with the same character.
A substring is a contiguous non-empty sequence of characters within a string.
Example 1:
Input: s = "abcba"
Output: 7
Explanation:
The substrings of length 1 that start and end with the same letter are: "a", "b", "c", "b", and "a".
The substring of length 3 that starts and ends with the same letter is: "bcb".
The substring of length 5 that starts and ends with the same letter is: "abcba" .
Example 2:
Input: s = "abacad"
Output: 9 112131
Explanation:
The substrings of length 1 that start and end with the same letter are: "a", "b", "a", "c", "a", and "d" = 6.
The substrings of length 3 that start and end with the same letter are: "aba" and "aca" = 2.
The substring of length 5 that starts and ends with the same letter is: "abaca" = 1.
Example 3:
Input: s = "a"
Output: 1
Explanation:
The substring of length 1 that starts and ends with the same letter is: "a".
Constraints:
1 <= s.length <= 105
s consists only of lowercase English letters.
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