Find duplicates in an Array with values 1 to N using counting sort

Last Updated : 26 Jun, 2020

Given a constant array of N elements which contain elements from 1 to N – 1, with any of these numbers appearing any number of times.

Examples:

Input: N = 5, arr[] = {1, 3, 4, 2, 2}
Output: 2
Explanation:
2 is the number occurring more than once.

Input: N = 5, arr[] = {3, 1, 3, 4, 2}
Output: 3
Explanation:
3 is the number occurring more than once.

Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Naive Approach: The naive method is to first sort the given array and then look for adjacent positions of the array to find the duplicate number.

Time Complexity: O(N*log N)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above method the idea is to use the concept of Counting Sort. Since the range of elements in the array is known, hence we could use this sorting technique to improvise the time complexity.
The idea is to initialize another array(say count[]) with the same size N and initialize all the elements as 0. Then count the occurrences of each element of the array and update the count in the count[]. Print all the element whose count is greater than 1.

Below is the implementation of above approach:

C++

// C++ program for the above approach 
#include <bits/stdc++.h> 
using namespace std; 
  
// Function to find the duplicate 
// number using countig sort method 
int findDuplicate(int arr[], int n) 
{ 
    int countArr[n + 1], i; 
  
    // Initialize all the elements 
    // of the countArr to 0 
    for (i = 0; i <= n; i++) 
        countArr[i] = 0; 
  
    // Count the occurences of each 
    // element of the array 
    for (i = 0; i <= n; i++) 
        countArr[arr[i]]++; 
  
    bool a = false; 
  
    // Find the element with more 
    // than one count 
    for (i = 1; i <= n; i++) { 
  
        if (countArr[i] > 1) { 
            a = true; 
            cout << i << ' '; 
        } 
    } 
  
    // If unique elements are ther 
    // print "-1" 
    if (!a) 
        cout << "-1"; 
} 
  
// Driver Code 
int main() 
{ 
    // Given N 
    int n = 4; 
  
    // Given array arr[] 
    int arr[] = { 1, 3, 4, 2, 2 }; 
  
    // Function Call 
    findDuplicate(arr, n); 
    return 0; 
} 

Java

// Java program for the above approach 
import java.util.*; 
  
class GFG { 
  
    // Function to find the duplicate number 
    // using countig sort method 
    public static int
    findDuplicate(int arr[], int n) 
    { 
        int countArr[] = new int[n + 1], i; 
  
        // Initialize all the elements of the 
        // countArr to 0 
        for (i = 0; i <= n; i++) 
            countArr[i] = 0; 
  
        // Count the occurences of each 
        // element of the array 
        for (i = 0; i <= n; i++) 
            countArr[arr[i]]++; 
  
        bool a = false; 
  
        // Find the element with more 
        // than one count 
        for (i = 1; i <= n; i++) { 
  
            if (countArr[i] > 1) { 
                a = true; 
                cout << i << ' '; 
            } 
        } 
        if (!a) 
            System.out.println("-1"); 
    } 
  
    // Driver Code 
    public static void main(String[] args) 
    { 
        int n = 4; 
        int arr[] = { 1, 3, 4, 2, 2 }; 
  
        // Function Call 
        findDuplicate(arr, n); 
    } 
} 

Python3

# Python3 program for the above approach 
  
# Function to find the duplicate 
# number using countig sort method  
def findDuplicate(arr, n): 
  
    # Initialize all the elements 
    # of the countArr to 0 
    countArr = [0] * (n + 1) 
  
    # Count the occurences of each 
    # element of the array 
    for i in range(n + 1): 
        countArr[arr[i]] += 1
  
    a = False
  
    # Find the element with more 
    # than one count 
    for i in range(1, n + 1): 
        if(countArr[i] > 1): 
            a = True
            print(i, end = " ") 
  
    # If unique elements are there 
    # print "-1" 
    if(not a): 
        print(-1) 
  
# Driver code 
if __name__ == '__main__': 
  
    # Given N 
    n = 4
  
    # Given array arr[] 
    arr = [ 1, 3, 4, 2, 2 ] 
  
    # Function Call 
    findDuplicate(arr, n) 
  
# This code is contributed by Shivam Singh 

C#

// C# program for the above approach 
using System; 
  
class GFG{ 
  
// Function to find the duplicate number 
// using countig sort method 
static void findDuplicate(int []arr, int n) 
{ 
    int []countArr = new int[n + 1]; 
    int i; 
  
    // Initialize all the elements of the 
    // countArr to 0 
    for(i = 0; i <= n; i++) 
       countArr[i] = 0; 
  
    // Count the occurences of each 
    // element of the array 
    for(i = 0; i <= n; i++) 
       countArr[arr[i]]++; 
  
    bool a = false; 
  
    // Find the element with more 
    // than one count 
    for(i = 1; i <= n; i++)  
    { 
       if (countArr[i] > 1) 
       { 
           a = true; 
           Console.Write(i + " "); 
        } 
    } 
    if (!a) 
        Console.WriteLine("-1"); 
} 
  
// Driver Code 
public static void Main(String[] args) 
{ 
    int n = 4; 
    int []arr = { 1, 3, 4, 2, 2 }; 
  
    // Function Call 
    findDuplicate(arr, n); 
} 
} 
  
// This code is contributed by Amit Katiyar 