Let me talk about my train of thought. Put a+""first; Well, A is a dead string. I remember that C# supports string types accessed with array subscripts. Then a[0] is 1, a[ 1] is 2, and so on. Then put a[0] in A, and finally put B[] in groups of three. Of course, you need to use a loop. The number of cycles is determined by the length of []. It shouldn't be difficult to take the array length. Write it down yourself. It's not difficult, right