具有最高索引的数组中的最大值

Question

I need to find most common digit in array of ints, i would also like to have the highest index(number) of them, so if there is input like [11, 22, 33] then it will return 3 instead of 1. How can I achieve that in easy way?

static uint mostCommonDigit(uint[] n)
    {
        uint[] numbersFrequency = new uint[10];
        foreach(uint i in n)
        {
            uint a = i;
            if (a != 0)
            {
                while (a>0)
                {
                    uint d = a % 10;
                    a = a / 10;
                    numbersFrequency[d] += 1;
                } 
            }
        }
        uint max = numbersFrequency.Max();
        int index = Array.IndexOf(numbersFrequency, max);
        return (uint)index;
    }

Answer 1

You could convert each element of the list to a string and concatenate them. Then, you could count the occurences of each character in that string. By sorting by character count and then by character value, higher characters will be sorted first if they occur with the same frequency:

char MostCommonDigit(int[] list)
{
    return list.Aggregate("", (i, j) => $"{i}{j}")
        .GroupBy(c => c)
        .Select(
            g => new {
                Char = g.Key,
                Count = g.Count()
        })
        .OrderByDescending(x => x.Count)
        .ThenByDescending(x => x.Char)
        .First().Char;
}

So

Console.WriteLine(MostCommonDigit(new [] { 11, 22, 33 }));
Console.WriteLine(MostCommonDigit(new [] { 111, 22, 33 }));

prints

3
1

Answer 2

You can use this linq to get your position:

List<int> iii  = new List<int> { 11, 22, 33 };
int yyy2 = iii.IndexOf(iii.Last(y => y.ToString().GroupBy(c => c).Select(c => c.Count()).Max() == iii.Select(x => x.ToString().GroupBy(c => c).Select(c => c.Count()).Max()).Max())) + 1;

Answer 3

As far as "easy" way, here is another LINQ alternative :

static uint mostCommonDigit(uint[] n) => 
    (uint)string.Concat(n).GroupBy(c => c).Max(g => (g.Count(), g.Key - '0')).Key

string.Concat converts the array to string (for example "112233").

GroupBy groups the characters in the string by character (for example '1' => ['1', '1'], '2' => ['2', '2']).

The Max part is similar to ordering by the number of items in each group, then by the key of each group, and then getting the last item, but it avoids the sorting. The - '0' part converts the character key to integer.

It is probably few times slower than your solution due to the overhead from LINQ, but the difference will be in milliseconds and not noticable for such small arrays.