unilingo-test.js

/**
 * @param {string} input
 * @description Given a string, return the character that appears the most frequently based on the frequency of the characters.
 * @returns {string}
 */
function main(input) {
  if (!input || typeof input !== 'string') return '';

  /*
   * Count character frequencies
   *
   * Convert the input string into an object with the character as the key and the frequency as the value
   * {
   *   a: 3,
   *   b: 2,
   *   c: 1,
   * }
   */
  const charCount = input.split('').reduce((acc, char) => {
    acc[char] = (acc[char] || 0) + 1;
    return acc;
  }, {});

  /*
   * Find the maximum frequency of the characters
   */
  const maxFreq = Math.max(...Object.values(charCount));

  /*
   * If max frequency is 1, return empty string
   */
  if (maxFreq === 1) return ''; // handle "abc" or "" case

  /*
   * Collect all characters with max frequency
   */
  const result = Object.entries(charCount)
    .filter(([_, count]) => count === maxFreq)
    .map(([char]) => char)
    .join('');

  return result;
}

// Test cases
console.log(main('aaabba')); // "a"
console.log(main('ccddcd')); // "cd"
console.log(main('abc')); // ""
console.log(main('efege')); // "e"
console.log(main('hihji')); // "hi"
console.log(main('')); // ""
console.log(main('klmmlk')); // "klm"
console.log(main('noonn')); // "n"
console.log(main('rrrtttrrrzzz')); // "r"