find-all-numbers-disappeared-in-an-array.java

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