public class DayCode {    public static void main(String[] args) {        int[] nums = new int[]{4,3,2,7,8,2,3,1};        List<Integer> ans = new DayCode().findDisappearedNumbers(nums);        System.out.println("ans is " + ans);    }
    /**     * https://leetcode-cn.com/problems/find-all-numbers-disappeared-in-an-array/     * @param nums     * @return     */    public List<Integer> findDisappearedNumbers(int[] nums) {        List<Integer> ans = new ArrayList<>();        int[] arr = new int[nums.length + 1];        for (int num : nums) {            arr[num] = 1;        }        for (int i = 1; i < arr.length; i++) {            if (arr[i] == 0) {                ans.add(i);            }        }        return ans;    }
    /**     *     * @param nums     * @return     */    public List<Integer> findDisappearedNumbers2(int[] nums) {        List<Integer> ans = new ArrayList<>();        int n = nums.length;        for (int num : nums) {            int x = (num - 1) % n;            nums[x] += n;        }        for (int i = 0; i < n; i++) {            if (nums[i] <= n) {                ans.add(i + 1);            }        }
        return ans;    }}