Find All Duplicates in an Array

Given an array of integers, 1 ≤ a[i] ≤n(n= size of array), some elements appeartwiceand others appearonce.

Find all the elements that appeartwicein this array.

Could you do it without extra space and in O(n) runtime?

EXAMPLE:

Input:
[4,3,2,7,8,2,3,1]

Output:
[2,3]

Solution:

If the number at position xxx is already negative, then the xxx must appeared twice.

public class Solution {
    public List<Integer> findDuplicates(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) ret.add(idx+1);
            nums[idx] = -nums[idx];
        }
        return ret;
    }
}