public static int sockMerchant(int n, List<int> ar)
    {
        int pairCount = 0;
        
        var groupList = ar.Order()
            .GroupBy(e => e)
            .Select(e => new { SockId = e.Key, Count = e.Count() })
        .ToList();
        
        foreach(var sockGroup in groupList)
        {
            pairCount += sockGroup.Count / 2;
        }
        
        return pairCount;
    }