LeetCode

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]

正负标记，不影响原先值。

public class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> arr = new ArrayList<Integer>();
        if (nums == null)
            return arr;
        for (int i=0; i<nums.length; i++) {
            int index = abs(nums[i]) - 1;
            if (nums[index] < 0)
                arr.add(abs(nums[i]));
            else
                nums[index] = -nums[index];
        }
        return arr;
    }
    
    private int abs(int a) {
        return a>0?a:-a;
    }
}