Leetcode 442. Find All Duplicates in an Array

/**
 * As described in the problem, 
 * 1 ≤ a[i] ≤ n (n = size of array)
 * therefore, we can use nums[nums[i]] = -nums[nums[i]] to help us which elements 
 * appear twice. Note that elements in the array appear at most twice. 
 */ 
public class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        int index;
        List<Integer> res = new ArrayList<Integer>();
        
        for (int i=0; i<nums.length; i++) {
            index = Math.abs(nums[i]);
            if (nums[index-1] < 0) res.add(index);
            else nums[index-1] = -nums[index-1];
        }
        return res;
    }
}

Similar problem, Leetcode 448. Find All Numbers Disappeared in an Array