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Find the smallest positive number missing from an unsorted array | Set 3

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  • Difficulty Level : Easy
  • Last Updated : 02 May, 2022

Given an unsorted array with both positive and negative elements. The task is to find the smallest positive number missing from the array.

Examples: 

Input: arr[] = {2, 3, 7, 6, 8, -1, -10, 15} 
Output: 1

Input: arr[] = { 2, 3, -7, 6, 8, 1, -10, 15 } 
Output: 4

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

Approach : 
We have already discussed some of the techniques to find the smallest positive number missing from an unsorted array.
Find the smallest positive number missing from an unsorted array | Set 1 
Find the smallest positive number missing from an unsorted array | Set 2
Here, we use set to store all positive integers and find the first missing positive integer. 

C++




// CPP program to find the smallest
// positive missing number
#include <bits/stdc++.h>
using namespace std;
 
// Function to find the smallest
// positive missing number
int findMissingPositive(int arr[], int n)
{
    // Default smallest Positive Integer
    int m = 1;
 
    // Store values in set which are
    // greater than variable m
    set<int> x;
 
    for (int i = 0; i < n; i++) {
        // Store value when m is less than
        // current index of given array
        if (m < arr[i]) {
            x.insert(arr[i]);
        }
        else if (m == arr[i]) {
            // Increment m when it is equal
            // to current element
            m = m + 1;
 
            while (x.count(m)) {
                x.erase(m);
 
                // Increment m when it is one of the
                // element of the set
                m = m + 1;
            }
        }
    }
 
    // Return the required answer
    return m;
}
 
// Driver code
int main()
{
    int arr[] = { 2, 3, -7, 6, 8, 1, -10, 15 };
 
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // Function call
    cout << findMissingPositive(arr, n);
 
    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 findMissingPositive(int arr[], int n)
{
    // Default smallest Positive Integer
    int m = 1;
 
    // Store values in set which are
    // greater than variable m
    HashSet<Integer> x = new HashSet<Integer>();
 
    for (int i = 0; i < n; i++)
    {
        // Store value when m is less than
        // current index of given array
        if (m < arr[i])
        {
            x.add(arr[i]);
        }
        else if (m == arr[i])
        {
            // Increment m when it is equal
            // to current element
            m = m + 1;
 
            while (x.contains(m))
            {
                x.remove(m);
 
                // Increment m when it is one of the
                // element of the set
                m = m + 1;
            }
        }
    }
 
    // Return the required answer
    return m;
}
 
// Driver code
public static void main(String[] args)
{
    int arr[] = { 2, 3, -7, 6, 8, 1, -10, 15 };
 
    int n = arr.length;
 
    // Function call
    System.out.println(findMissingPositive(arr, n));
}
}
 
// This code is contributed by 29AjayKumar

Python3




# Python 3 program to find the smallest
# positive missing number
 
# Function to find the smallest
# positive missing number
def findMissingPositive(arr, n):
     
    # Default smallest Positive Integer
    m = 1
 
    # Store values in set which are
    # greater than variable m
    x = []
    for i in range(n):
         
        # Store value when m is less than
        # current index of given array
        if (m < arr[i]):
            x.append(arr[i])
     
        elif (m == arr[i]):
             
            # Increment m when it is equal
            # to current element
            m = m + 1
 
            while (x.count(m)):
                x.remove(m)
 
                # Increment m when it is one of the
                # element of the set
                m = m + 1
 
    # Return the required answer
    return m
 
# Driver code
if __name__ == '__main__':
    arr = [2, 3, -7, 6, 8, 1, -10, 15]
 
    n = len(arr)
 
    # Function call
    print(findMissingPositive(arr, n))
 
# This code is contributed by
# Surendra_Gangwar

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 findMissingPositive(int []arr, int n)
{
    // Default smallest Positive Integer
    int m = 1;
 
    // Store values in set which are
    // greater than variable m
    HashSet<int> x = new HashSet<int>();
 
    for (int i = 0; i < n; i++)
    {
        // Store value when m is less than
        // current index of given array
        if (m < arr[i])
        {
            x.Add(arr[i]);
        }
         
        else if (m == arr[i])
        {
            // Increment m when it is equal
            // to current element
            m = m + 1;
 
            while (x.Contains(m))
            {
                x.Remove(m);
 
                // Increment m when it is one of the
                // element of the set
                m = m + 1;
            }
        }
    }
 
    // Return the required answer
    return m;
}
 
// Driver code
public static void Main(String[] args)
{
    int []arr = { 2, 3, -7, 6, 8, 1, -10, 15 };
 
    int n = arr.Length;
 
    // Function call
    Console.WriteLine(findMissingPositive(arr, n));
}
}
 
// This code is contributed by Rajput-Ji

Javascript




<script>
 
// JavaScript program to find the smallest
// positive missing number
 
// Function to find the smallest
// positive missing number
function findMissingPositive(arr, n)
{
    // Default smallest Positive integer
    let m = 1;
 
    // Store values in set which are
    // greater than variable m
    let x = new Set();
 
    for (let i = 0; i < n; i++) {
        // Store value when m is less than
        // current index of given array
        if (m < arr[i]) {
            x.add(arr[i]);
        }
        else if (m == arr[i]) {
            // Increment m when it is equal
            // to current element
            m = m + 1;
 
            while (x.has(m)) {
                x.delete(m);
 
                // Increment m when it is one of the
                // element of the set
                m = m + 1;
            }
        }
    }
 
    // Return the required answer
    return m;
}
 
// Driver code
 
let arr = [2, 3, -7, 6, 8, 1, -10, 15];
 
let n = arr.length;
 
// Function call
document.write(findMissingPositive(arr, n));
 
 
// This code is contributed by _saurabh_jaiswal
 
</script>

Output: 

4

 

Time Complexity: O(nlog(n))

Auxiliary Space: O(n)


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