LeetCode-442. Find All Duplicates in an Array C#

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: Similar to  Leetcode-448

Mark the value of already apeared index to negative, then if the value appeared second time the index value should be negative, add it to the result list.

 

public class Solution {
    public IList<int> FindDuplicates(int[] nums) {
        int sum = 0;
        IList<int> result = new List<int>();
        for (int i = 0; i<nums.Length; i++)
        {
            int index = Math.Abs(nums[i])-1;
            if(nums[index]<0)
            {
                result.Add(Math.Abs(nums[i]));
            }
            else
            {
                nums[index] = (-1)*nums[index];
            }
        }
        return result;
        
    }
}