442. Find All Duplicates in an Array

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]

Solution：

思路： 因为都是正数，可以置相同值的index位置为负
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

public class Solution {
    // when find a number i, flip the number at position i-1 to negative. 
    // if the number at position i-1 is already negative, i is the number that occurs twice.
    
    public List findDuplicates(int[] nums) {
        List res = new ArrayList<>();
        for (int i = 0; i < nums.length; ++i) {
            int index = Math.abs(nums[i]) - 1;
            if (nums[index] < 0)
                res.add(Math.abs(index + 1));
            nums[index] = -nums[index];
        }
        return res;
    }
}