Algorithm 2 Reverse  operator

Input:$|x_0{x}_1\dots x_{n-1}{x}_n\rangle$ Output:$|x_n{x}_{n-1}\dots x_1{x}_0\rangle$  1: SWAP($x_0,x_n$)  2: …  3: SWAP($x_{n/2},x_{n/2+1}$)