[LeetCode] 448. Find All Numbers Disappeared in an Array

ProblemGiven an array of integers where 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.Find all the elements of [1, n] inclusive that do not appear in this array.Could you do it without extra space and in O(n) runtime? You may assume the returned list does not count as extra space.Example:Input:[4,3,2,7,8,2,3,1]Output:[5,6]Solutionclass Solution { public List<Integer> findDisappearedNumbers(int[] nums) { List<Integer> res = new ArrayList<>(); if (nums == null || nums.length == 0) return res; for (int i = 0; i < nums.length; i++) { int index = nums[i] > 0 ? nums[i]-1 : -nums[i]-1; if (nums[index] < 0) continue; nums[index] *= -1; } for (int i = 0; i < nums.length; i++) { if (nums[i] > 0) res.add(i+1); } return res; }}