##### Welcome to Subscribe On Youtube

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.

• class Solution {
public long numberOfSubstrings(String s) {
int[] cnt = new int;
long ans = 0;
for (int i = 0; i < s.length(); ++i) {
int j = s.charAt(i) - 'a';
++cnt[j];
ans += cnt[j];
}
return ans;
}
}

• class Solution {
public:
long long numberOfSubstrings(string s) {
int cnt{};
long long ans = 0;
for (char& c : s) {
ans += ++cnt[c - 'a'];
}
return ans;
}
};

• class Solution:
def numberOfSubstrings(self, s: str) -> int:
cnt = Counter()
ans = 0
for c in s:
cnt[c] += 1
ans += cnt[c]
return ans


• func numberOfSubstrings(s string) (ans int64) {
cnt := int{}
for _, c := range s {
c -= 'a'
cnt[c]++
ans += int64(cnt[c])
}
return ans
}