451. 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

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.

Approach

Solution

/**
 * @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("");
};

Explanation