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Problem #4

This problem was asked by Stripe.

Given an array of integers, find the first missing positive integer in linear time and constant space. In other words, find the lowest positive integer that does not exist in the array. The array can contain duplicates and negative numbers as well.

For example, the input [3, 4, -1, 1] should give 2. The input [1, 2, 0] should give 3.

You can modify the input array in-place.

My solution

using System;
using System.Collections.Generic;
using System.Linq;

namespace DailyCodingProblem.Solutions.Problem04
{
    public class Solution
    {
        public static void Test()
        {
            var input = Console.ReadLine()
                .Split(' ')
                .Select(int.Parse)
                .ToHashSet();

            var minPositiveNumber = GetMinimalPositiveNumber(input);

            Console.WriteLine(minPositiveNumber);
        }

        public static int GetMinimalPositiveNumber(HashSet<int> input)
        {
            var minPositiveNumber = 1;
            while (true)
            {
                if (!input.Contains(minPositiveNumber))
                {
                    break;
                }

                minPositiveNumber++;
            }

            return minPositiveNumber;
        }
    }
}

Solutions are posted in my Github account