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

08 Jun, 2021
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++
Java
Python3
C#
Javascript
// C++ program to find the smallest// positive missing number #include <bits/stdc++.h>using namespace std; // Function to find the smallest positive// missing numberint 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 codeint 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 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 program to find the smallest# positive missing number # Function to find the smallest positive# missing numberdef 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 codeif __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# program to find the smallest// positive missing numberusing 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
<script> // JavaScript program to find the smallest// positive missing number  // Function to find the smallest positive// missing numberfunction 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>
Smallest positive missing number is : 2
 
Time Complexity: O(N) Auxiliary Space: O(N) 
29AjayKumar
ankthon
princi singh
Akanksha_Rai
gfgking
cpp-unordered_map
frequency-counting
Arrays
Heap
Arrays
Heap
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