char[ ] f(char[ ] a1, char[ ] a2) 
        {
            if (a1.Length<0 && a2.Length<0)
                return null;
            else if(a1.Length<0 && a2.Length>0)
                return a2;
            else if(a2.Length<0 && a1.Length>0)
                return a1;
            else
            {
                int j=0;
                for(int i=0;i<a1.Length;i++,j++)
                {
                    if(a1[i]>a2[j])
                        return a1;
                    if(a1[i]<a2[j])
                        return a2;
                }
                if(j<a2.Length)
                    return a2;
                else
                    return a1;

            }
        }

    ConsoleApplication11.Class1 t= new ConsoleApplication11.Class1( );
            System.Console.WriteLine(t.f(new char[]{'a', 'b', 'c'},{'a'}));
系统提示最后一句有错，错在什么地方"请高手指点