LC 442. 数组中重复的数据

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