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#1930 - Unique Length-3 Palindromic Subsequences
Problem Description
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.
A palindrome is a string that reads the same forwards and backwards.
A 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".
Solution
/**
* @param {string} s
* @return {number}
*/
var countPalindromicSubsequence = function(s) {
let result = 0;
for (let i = 0; i < 26; ++i) {
const char = String.fromCharCode(i + 97);
const left = s.indexOf(char);
const right = s.lastIndexOf(char);
if (left !== -1 && right !== -1 && left < right) {
result += new Set(s.substring(left + 1, right)).size;
}
}
return result;
};