[LeetCode] Sort Characters By Frequency

Problem: 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: 

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.

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.

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.

Approach: We can use custom comparer while applying sorting on this string.

Implementation in C#:

    public string FrequencySort(string s) 

    {

        char[] arr = s.ToCharArray();

        Dictionary<char, int> count = new Dictionary<char, int>();

        foreach (char ch in arr)

        {

            if (!count.ContainsKey(ch))

            {

                count[ch] = 0;

            }

            ++count[ch];

        }

        Array.Sort(arr, (a, b) => {

            if (count[b] == count[a])

            {

                return b.CompareTo(a);

            }      

            return count[b].CompareTo(count[a]); 

        });

        return new string(arr);

    }

Complexity: O(nlogn)