First Duplicate

Solution to the "First Duplicate" problem in C#.

Problem

Given an array that contains only numbers in the range 1 to array.Length, find the first duplicate number for which the second occurrence has the minimal index. If there is no such number, return -1.

Note: Write a solution with O(n) time complexity and O(1) additional space complexity.

Example

Input:

[2, 3, 3, 1, 5, 2]

Output:

3

There are 2 duplicates: 2 and 3. the second occurrence of 3 has smaller index (position 2 of the array) than the second occurrence of 2 (position 5 of the array). Hence the first duplicate is 3.

Input:

[2, 4, 3, 5, 1]

Output:

- 1

There is not duplicated numbers in the array.

Solution

using System;
using System.Collections.Generic;

namespace FirstDuplicate
{
    class Program
    {
        public static int FirstDuplicate(int[] a)
        {

            HashSet<int> set = new HashSet<int>();

            for(var i = 0; i <= a.Length - 1; i++)
            {
                if (set.Contains(a[i]))
                {
                    return a[i];
                }

                set.Add(a[i]);
            }

            return -1;
        }
        static void Main(string[] args)
        {
            int[] arr = {2, 3, 3, 1, 5, 2};
            Console.Write(FirstDuplicate(arr));
            Console.ReadLine();

        }
    }
}

Constraints

1 <= array.Length <= 10^5
1 <= array[i] <= a.length