leetcode-go/medium/667.beautiful-arrangement-ii.go

func constructArray(n int, k int) []int {
	res := make([]int, n)
	res[0] = 1
	l, r, i, f := 2, n, 1, 0
	for ; k != 1; k-- {
		if f == 0 {
			res[i] = r
			r--
		} else {
			res[i] = l
			l++
		}
		i++
		f ^= 1
	} // To increase k, we have:
	//                        _______              _____                ___
	//   1 2 3 4 5 (k=1) -> 1 5 4 3 2 (k=2) -> 1 5 2 3 4 (k=3) -> 1 5 2 4 3 (k=4)
	if f == 0 {
		for ; l <= r; l++ {
			res[i] = l
			i++
		}
	} else {
		for ; l <= r; r-- {
			res[i] = r
			i++
		}
	}
	return res
}