Find duplicates in array

The problem offered a unique constraint, that the values in the given array lie between 1 to n, where n-1 is the length of given array.
class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> res=new ArrayList<>();
        if(nums.length<=1) return res;
        for(int i=0; i<nums.length; i++){
    if (nums[Math.abs(nums[i])-1] > 0) {
        nums[Math.abs(nums[i])-1]= -1 * nums[Math.abs(nums[i])-1];
    } else {
      res.add(Math.abs(nums[i]));
    }
  
        }
        return res;
    }
}
# Status: solved