func findDisappearedNumbers(nums []int) (res []int) {
	n := len(nums)
	for i := 0; i < n; i++ {
		if idx := abs(nums[i]) - 1; 0 < nums[idx] {
			nums[idx] = -nums[idx]
		}
	}
	for i := 0; i < n; i++ {
		if 0 < nums[i] {
			res = append(res, i+1)
		}
	}
	return
}

func abs(x int) int {
	if x < 0 {
		return -x
	}
	return x
}

func findDisappearedNumbersOn(nums []int) (res []int) { // O(n) extra space
	n := len(nums)
	set := make([]bool, n+1)
	for _, i := range nums {
		set[i] = true
	}
	for i := 1; i <= n; i++ {
		if !set[i] {
			res = append(res, i)
		}
	}
	return
}