leetcode-go/medium/526.beautiful-arrangement.go

func countArrangement(N int) (cnt int) {
	dfs(N, 1, 0, &cnt)
	return cnt
}

func dfs(N int, i int, used int, cnt *int) {
	if N < i {
		(*cnt)++
		return
	}
	for a := 1; a <= N; a++ {
		if mask := 1 << uint(a); used&mask == 0 && (a%i == 0 || i%a == 0) {
			used |= mask
			dfs(N, i+1, used, cnt)
			used ^= mask // Backtracking
		}
	}
}