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Given a string s, return the number of unique palindromes of length three that are a subsequence of s.

Note that even if there are multiple ways to obtain the same subsequence, it is still only counted once.

palindrome is a string that reads the same forwards and backwards.

subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

• For example, "ace" is a subsequence of "abcde".

Example 1:

Input: s = "aabca"
Output: 3
Explanation: The 3 palindromic subsequences of length 3 are:
- "aba" (subsequence of "aabca")
- "aaa" (subsequence of "aabca")
- "aca" (subsequence of "aabca")


Example 2:

Input: s = "adc"
Output: 0
Explanation: There are no palindromic subsequences of length 3 in "adc".


Example 3:

Input: s = "bbcbaba"
Output: 4
Explanation: The 4 palindromic subsequences of length 3 are:
- "bbb" (subsequence of "bbcbaba")
- "bcb" (subsequence of "bbcbaba")
- "bab" (subsequence of "bbcbaba")
- "aba" (subsequence of "bbcbaba")


Constraints:

• 3 <= s.length <= 105
• s consists of only lowercase English letters.

## Solution: Enumerate first character of a palindrome

For a length 3 palindrome, we just need to enumerate the first character c.
We found the first and last occurrence of c in original string and scan the middle part to see how many unique characters there.

e.g. aabca
Enumerate from a to z, looking for a*a, b*b, …, z*z.
For a*a, aabca, we found first and last a, in between is abc, which has 3 unique letters.
We can use a hastable or a bitset to track unique letters.

Time complexity: O(26*n)
Space complexity: O(1)

## C++

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