leetcode/work/Medium/Hash Table/451.sort-characters-by-frequency.js

/*
 * 451. Sort Characters By Frequency
 * Difficulty: Medium
 * https://leetcode.com/problems/sort-characters-by-frequency/
 *
 * ──────────────────────────────────────────────────
 *
 * Given a string s, sort it in decreasing order based on the frequency
 * of the characters. The frequency of a character is the number of times
 * it appears in the string.
 *
 * Return the sorted string. If there are multiple answers, return any
 * of them.
 *
 *
 *
 * Example 1:
 *
 * Input: s = "tree"
 * Output: "eert"
 * Explanation: 'e' appears twice while 'r' and 't' both appear once.
 * So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also
 * a valid answer.
 *
 * Example 2:
 *
 * Input: s = "cccaaa"
 * Output: "aaaccc"
 * Explanation: Both 'c' and 'a' appear three times, so both "cccaaa"
 * and "aaaccc" are valid answers.
 * Note that "cacaca" is incorrect, as the same characters must be
 * together.
 *
 * Example 3:
 *
 * Input: s = "Aabb"
 * Output: "bbAa"
 * Explanation: "bbaA" is also a valid answer, but "Aabb" is incorrect.
 * Note that 'A' and 'a' are treated as two different characters.
 *
 *
 *
 * Constraints:
 *
 * 	• 1 <= s.length <= 5 * 10^5
 *
 * 	• s consists of uppercase and lowercase English letters and digits.
 */

/**
 * @param {string} s
 * @return {string}
 */
var frequencySort = function (s) {
  // count the frequency of each character
  const freq = {};
  for (let c of s) freq[c] = (freq[c] || 0) + 1;

  // sort the characters by frequency
  return s
    .split("")
    .sort((a, b) => freq[b] - freq[a] || a.localeCompare(b))
    .join("");
};