Find All Numbers Disappeared in an Array

Given 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]

Solution:

The idea is to use the indices of elements as the marks and set the corresponding elements to negative if met in the array.

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