1930.js

/** ------------------------------------------------------------------------------
 * 
 * 1930. Unique Length-3 Palindromic Subsequences
 * Topics: String, Hash Table
 * https://leetcode.com/problems/unique-length-3-palindromic-subsequences/?envType=daily-question&envId=2025-01-04
 * 
 ------------------------------------------------------------------------------ */
/**
 * @param {string} s
 * @return {number}
 */
var countPalindromicSubsequence = function (s) {
  let answer = 0
  const first = Array(26).fill(-1)
  const last = Array(26).fill(-1)

  for (let i = 0; i < s.length; i++) {
    const j = s.charCodeAt(i) - "a".charCodeAt(0)
    if (first[j] === -1) {
      first[j] = i
    }
    last[j] = i
  }

  for (let i = 0; i < 26; i++) {
    if (first[i] === -1 || first[i] === last[i]) continue

    const l = first[i],
      r = last[i]
    const mids = new Set()
    for (let j = l + 1; j < r; j++) {
      mids.add(s[j])
    }
    answer += mids.size
  }
  return answer
}

/**
 * indexOf, lastIndexOf 사용한 간단풀이
 */
var countPalindromicSubsequence = function (s) {
  let answer = 0
  const uniq = new Set(s)

  for (const c of uniq) {
    const start = s.indexOf(c)
    const end = s.lastIndexOf(c)

    if (start < end) {
      answer += new Set(s.slice(start + 1, end)).size
    }
  }
  return answer
}

console.log(countPalindromicSubsequence("aabca"))