Find the smallest positive number missing from an unsorted array : Hashing Implementation

Given an unsorted array with both positive and negative elements including 0. The task is to find the smallest positive number missing from the array in O(N) time. 
Examples:

Input: arr[] = {-5, 2, 0, -1, -10, 15}
Output: 1

Input: arr[] = {0, 1, 2, 3, 4, 5}
Output: 6

Input: arr[] = {1, 1, 1, 0, -1, -2}
Output: 2

We can use hashing to solve this problem. The idea is to build a hash table of all positive elements in the given array. Once the hash table is built. We can look in the hash table for all positive integers, starting from 1. As soon as we find a number which is not there in the hash table, we return it. 
We can use the unordered_map in C++ to implement the hash which allows performing look-up operation in almost O(1) time complexity.
Below is the implementation of the above approach: 

C++

// C++ program to find the smallest
// positive missing number

#include <bits/stdc++.h>
using namespace std;

// Function to find the smallest positive
// missing number
int missingNumber(int a[], int n)
{
    // Declaring an unordered_map
    unordered_map<int, int> mp;

    // if array value is positive
    // store it in map
    for (int i = 0; i < n; i++) {
        if (a[i] > 0)
            mp[a[i]]++;
    }

    // index value set to 1
    int index = 1;

    // Return the first value starting
    // from 1 which does not exists in map
    while (1) {
        if (mp.find(index) == mp.end()) {
            return index;
        }

        index++;
    }
}

// Driver code
int main()
{
    int a[] = { 1, 1, 1, 0, -1, -2 };
    int size = sizeof(a) / sizeof(a[0]);

    cout << "Smallest positive missing number is : "
         << missingNumber(a, size) << endl;

    return 0;
}

Java

// Java program to find the smallest
// positive missing number

import java.util.*;

class GFG
{

    // Function to find the smallest positive
    // missing number
    static int missingNumber(int a[], int n)
    {
        // Declaring an unordered_map
        Map<Integer, Integer> mp = new LinkedHashMap<>();

        // if array value is positive
        // store it in map
        for (int i = 0; i < n; i++)
        {
            if (a[i] > 0)
            {
                mp.put(a[i], mp.get(a[i]) == null ? 1 : mp.get(a[i]) + 1);
            }
        }

        // index value set to 1
        int index = 1;

        // Return the first value starting
        // from 1 which does not exists in map
        while (true)
        {
            if (!mp.containsKey(index))
            {
                return index;
            }

            index++;
        }
    }

    // Driver code
    public static void main(String[] args)
    {
        int a[] = {1, 1, 1, 0, -1, -2};
        int size = a.length;

        System.out.println("Smallest positive missing number is : "
                + missingNumber(a, size));
    }
}

// This code has been contributed by 29AjayKumar

Python3

# Python3 program to find the smallest
# positive missing number

# Function to find the smallest positive
# missing number
def missingNumber(a, n) :

    # Declaring an unordered_map
    mp = dict();

    # if array value is positive
    # store it in map
    for i in range(n) :
        if (a[i] > 0) :
            if a[i] not in mp.keys() :
                mp[a[i]] = 0

            mp[a[i]] += 1

    # index value set to 1
    index = 1;

    # Return the first value starting
    # from 1 which does not exists in map
    while (1) :
        if (index not in mp.keys()) :
            return index;


        index += 1;


# Driver code
if __name__ == "__main__" :

    a = [ 1, 1, 1, 0, -1, -2 ];
    size = len(a);

    print("Smallest positive missing number is : ",missingNumber(a, size));

# This code is contributed by AnkitRai01

C#

// C# program to find the smallest
// positive missing number
using System;
using System.Collections.Generic;

class GFG
{

    // Function to find the smallest positive
    // missing number
    static int missingNumber(int []a, int n)
    {
        // Declaring an unordered_map
        Dictionary<int,int> m = new Dictionary<int,int>();

        // if array value is positive
        // store it in map
        for (int i = 0; i < n; i++)
        {
            if (a[i] > 0)
            {
                if(m.ContainsKey(a[i]))
                {
                    var val = m[a[i]];
                    m.Remove(a[i]);
                    m.Add(a[i], val + 1);
                }
                else
                {
                    m.Add(a[i], 1);
                }
            }
        }

        // index value set to 1
        int index = 1;

        // Return the first value starting
        // from 1 which does not exists in map
        while (true)
        {
            if (!m.ContainsKey(index))
            {
                return index;
            }

            index++;
        }
    }

    // Driver code
    public static void Main(String[] args)
    {
        int []a = {1, 1, 1, 0, -1, -2};
        int size = a.Length;

        Console.WriteLine("Smallest positive missing number is : "
                + missingNumber(a, size));
    }
}

// This code is contributed by Princi Singh

Javascript

<script>

// JavaScript program to find the smallest
// positive missing number


// Function to find the smallest positive
// missing number
function missingNumber(a, n) {
    // Declaring an unordered_map
    let mp = new Map();

    // if array value is positive
    // store it in map
    for (let i = 0; i < n; i++) {
        if (a[i] > 0) {
            mp[a[i]]++;
            if (mp.has(a[i])) {
                mp.set(a[i], mp.get(a[i]) + 1)
            } else {
                mp.set(a[i], 1)
            }
        }
    }

    // index value set to 1
    let index = 1;

    // Return the first value starting
    // from 1 which does not exists in map
    while (1) {
        if (!mp.has(index)) {
            return index;
        }

        index++;
    }
}

// Driver code

let a = [1, 1, 1, 0, -1, -2];
let size = a.length;

document.write("Smallest positive missing number is : " +
missingNumber(a, size) + "<br>");


// This code is contributed by gfgking

</script>

Output:

Smallest positive missing number is : 2

Time Complexity: O(N) 
Auxiliary Space: O(N)