448. Find All Numbers Disappeared in an Array

class Solution {
    public List<Integer> findDisappearedNumbers(int[] nums) {
        List<Integer> ans = new ArrayList<Integer>();
        
        for(int i = 0; i < nums.length; i++) {
            int j = Math.abs(nums[i]) - 1;
            if(nums[j] > 0) {
                nums[j] = -nums[j];
            }
        }
        
        for(int i = 0; i < nums.length; i++) {
            if(nums[i] > 0) {
                ans.add(i+1);
            }
        }
        return ans;
    }
}