Leecode - 442. Find All Duplicates in an Array (Java, Javascript,C#,C,C++,Ruby)

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]
//Java
class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        for(int i = 0; i < nums.length; i++)
        {                
            int index = Math.abs(nums[i])-1;
            // if(nums[index] < 0 && !list.contains(index+1))
            if(nums[index] < 0)
            {
                list.add(Math.abs(nums[i]));
            }
            else
            {
                nums[index] = -nums[index];
            }
        }
        // if(list.size()!=0)
        // {
        //     // list.add(-1);
        //     Collections.sort(list);
        // }
        return list;
    }
}
//Javascript
var findDuplicates = function(nums) {
    let list = [];
    
    for(let i = 0; i < nums.length; i++)
    {                
        let index = Math.abs(nums[i])-1;
        if(nums[index] < 0)
        {
            list.push(index+1);
        }else
        {
            nums[index] = -nums[index];
        }
    }
    return list;
};
//Csharp
public class Solution {
    public IList<int> FindDuplicates(int[] nums) {
        IList<int> list= new List<int>();
        for(int i = 0; i < nums.Length; i++)
        {                
            int index = Math.Abs(nums[i])-1;
            if(nums[index] < 0)
            {
                list.Add(Math.Abs(nums[i]));
            }else
            {
                nums[index] = -nums[index];
            }
        }
        return list;
    }
}
//C
int* findDuplicates(int* nums, int numsSize, int* returnSize){
int* list = (int*)malloc(numsSize*sizeof(int));
int list_idx = 0;
    
    for(int i = 0; i < numsSize; i++)
    {                
        int index = abs(nums[i])-1;
        if(nums[index] < 0)
        {
            list[list_idx++]= abs(nums[i]);
        }else
        {
            nums[index] = -nums[index];
        }
    }
    *returnSize = list_idx;
    return list;
}
//C++
class Solution {
public:
    vector<int> findDuplicates(vector<int>& nums) {
        vector<int>  list;
        for(int i = 0; i < nums.size(); i++)
        {                
            int index = abs(nums[i])-1;
            // if(nums[index] < 0 && !list.contains(index+1))
            if(nums[index] < 0)
            {
                list.push_back(abs(nums[i]));
            }
            else
            {
                nums[index] = -nums[index];
            }
        }
        return list;
    }
};
//Ruby
def find_duplicates(nums)
    list = [];    
    nums.each.each.with_index do |num, i|               
        index = num.abs()-1;
        if nums[index]  < 0
            list.push(num.abs);
        else
            nums[index] = -nums[index];
        end
    end
    return list;
end