Solution.cs

using System;
using System.Globalization;
using System.Collections.Generic;
using System.Linq;
using System.Text;

public class Comparer: IComparer<string>
{
    public int Compare(string left, string right)
    {
        return Compare(left, right, 0, 0);
    }
    
    private int Compare(string left, string right, int lBegin, int rBegin)
    {
        var len = Math.Min(left.Length - lBegin, right.Length - rBegin);
        for (var i = 0; i < len; ++i)
        {
            if (left[lBegin + i] != right[rBegin + i])
            {
                return left[lBegin + i] < right[rBegin + i] ? -1 : 1;
            }
        }
        
        if (left.Length - lBegin == right.Length - rBegin)
        {
            return 0;
        }
        if (left.Length - lBegin > right.Length - rBegin)
        {
            return Compare(left, right, lBegin + len, rBegin);
        }
        else
        {
            return Compare(left, right, lBegin, rBegin + len);
        }
    }
}

public class Solution {
    public string LargestNumber(int[] nums) {
        var sb = new StringBuilder();
        var strs = nums.Select(n => n.ToString(CultureInfo.InvariantCulture)).OrderByDescending(s => s, new Comparer());
        
        var nonZeroOccurred = false;
        foreach (var str in strs)
        {
            if (!nonZeroOccurred && str == "0") continue;
            sb.Append(str);
            nonZeroOccurred = true;
        }
        return sb.Length == 0 ? "0" : sb.ToString();
    }
}