Formatted question description: https://leetcode.ca/all/2083.html

# 2083. Substrings That Begin and End With the Same Letter (Medium)

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
Explanation:
The substrings of length 1 that start and end with the same letter are: "a", "b", "a", "c", "a", and "d".
The substrings of length 3 that start and end with the same letter are: "aba" and "aca".
The substring of length 5 that starts and ends with the same letter is: "abaca".


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.

Companies:

Related Topics:
Hash Table, Math, String, Counting, Prefix Sum

Similar Questions:

## Solution 1.

// OJ: https://leetcode.com/problems/substrings-that-begin-and-end-with-the-same-letter/
// Time: O(N)
// Space: O(1)
class Solution {
public:
long long numberOfSubstrings(string s) {
long long ans = 0, cnt[26] = {};
for (int i = 0; i < s.size(); ++i) {
ans += ++cnt[s[i] - 'a'];
}
return ans;
}
};