Find All Duplicates in an Array

题目地址：
https://leetcode.com/problems/find-all-duplicates-in-an-array/description/

题目：

Given an array of integers, 1 ≤ a[i] ≤ n (n = size of array), some elements appear twice and others appear once.

Find all the elements that appear twice in 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]

解题思路：
这道题就是将遇到的数字应该填的位置的数字置为相反数。

代码：

public List<Integer> findDuplicates(int[] nums) {
    List<Integer> rst = new ArrayList<>();
    for(int num : nums){
        int index = Math.abs(num);
        if(nums[index - 1] < 0){
            rst.add(index);
        }
        else{
            nums[index - 1] *= -1;
        }
    }
    return rst;
}