package main

import "fmt"

func grayCode(n int) (res []int) {
	num , ibook := 0 , make([]int,n,n)
	for i := range ibook {
		ibook[i] = -1
	}
	var depth func (floor int)
	depth = func(floor int) {
		if floor == n {
			res = append(res, num)
			return
		}
		inputV := 0
		floorV := 1 << (n - floor -1)
		for i := 0 ; i < 2 ; i ++ {
			if i == 0 {
				if ibook[floor] < 0 {
					ibook[floor] = 0
					inputV = 0
				}else if ibook[floor] == 0 {
					inputV = 0
				}else if ibook[floor] == 1 {
					inputV = 1
				}
			v := inputV * floorV
			num += v
			depth(floor + 1)
			num -= v
			}else {
				if inputV == 1 {
					ibook[floor] = 0
					depth(floor + 1)
				}else {
					ibook[floor] = 1
					v := floorV
					num += v
					depth(floor + 1)
					num -= v
				}
			}
		}
	}
	depth(0)
	return  res
}

func main() {
	fmt.Println(grayCode(3))
}