有2*n个盒子排成一行，其中有两个相邻的空盒，有n-1个盒子有符号 A ，有n-1个盒子有符号 B 。例如，n=5，并有初始配置如下： A B B A . . A B A B-2* n box row, wherein there are two adjacent empty box, n-1 of a box with the symbol ' A' , the symbol ' B'　n-1 th box. For example, n = 5, and the initial configuration is as follows: ABBA .. ABAB