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#1930 - Unique Length-3 Palindromic Subsequences

Medium • View on LeetCode • View on GitHub

String Hash Table Bit Manipulation Prefix Sum

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;
};

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