Gray码是一个长度为2n的序列。序列中无相同的原图，每个元素都是长度为n位的串，相邻元素恰好只有一位不同。用分治策略设计一个算法对任意的n构造相应的Gray码

The Gray code is a sequence of length 2n. There is no identical original image in the sequence. Each element is a string of length n bits, and the adjacent elements are exactly one bit different. Use the divide and conquer strategy to design an algorithm to construct the corresponding Gray code for any n.