How to arrange an array in decreasing order of frequency of each number?

Question

Input : {5, 13, 6, 5, 13, 7, 8, 6, 5}

Output : {5, 5, 5, 13, 13, 6, 6, 7, 8}

The question is to arrange the numbers in the array in

Answer 1

C#. It takes the most obvious approach: sort by frequency and ordinal.

Output: 5 5 5 13 13 6 6 7 8. Space: O(n). Time: O(n log n).

class Program
{
    class FreqAndOrdinal
    {
        public int Frequency;
        public int Ordinal;
        public FreqAndOrdinal(int freq, int ord)
        {
            this.Frequency = freq;
            this.Ordinal = ord;
        }
    }

    static int Compare(FreqAndOrdinal x, FreqAndOrdinal y)
    {
        int result = y.Frequency.CompareTo(x.Frequency);
        return result == 0 ? x.Ordinal.CompareTo(y.Ordinal) : result;
    }

    static void Main(string[] args)
    {
        int[] nums = new int[] { 5, 13, 6, 5, 13, 7, 8, 6, 5 };
        var freqLookup = new Dictionary<int, FreqAndOrdinal>(nums.Length);
        for (int i = 0; i < nums.Length; i++)
        {
            FreqAndOrdinal tmp;
            if (freqLookup.TryGetValue(nums[i], out tmp))
                ++tmp.Frequency;
            else
                freqLookup[nums[i]] = new FreqAndOrdinal(1, i);
        }

        Array.Sort(nums, (x,y) => Compare(freqLookup[x], freqLookup[y]));

        for (int i = 0; i < nums.Length; i++)
        {
            Console.Write(" {0}", nums[i]);
        }
        Console.ReadKey();
    }
}