Find duplicates in a given array when elements are not limited to a range

Given an array of n integers. The task is to print the duplicates in the given array. If there are no duplicates then print -1.
Examples:

Input: {2, 10,10, 100, 2, 10, 11,2,11,2}
Output: 2 10 11

Input: {5, 40, 1, 40, 100000, 1, 5, 1}
Output: 5 40 1

Note: The duplicate elements can be printed in any order.

Simple Approach: The idea is to use nested loop and for each element check if the element is present in the array more than once or not. If present, then store it in a Hash-map. Otherwise, continue checking other elements.

Below is the implementation of the above approach:

Java

// Java implementation of the
// above approach
 
import java.util.ArrayList;
 
public class GFG {
 
    // Function to find the Duplicates,
    // if duplicate occurs 2 times or
    // more than 2 times in
    // array so, it will print duplicate
    // value only once at output
    static void findDuplicates(
      int arr[], int len)
    {
 
        // initialize ifPresent as false
        boolean ifPresent = false;
 
        // ArrayList to store the output
        ArrayList<Integer> al = new ArrayList<Integer>();
 
        for (int i = 0; i < len - 1; i++) {
            for (int j = i + 1; j < len; j++) {
                if (arr[i] == arr[j]) {
                    // checking if element is
                    // present in the ArrayList
                    // or not if present then break
                    if (al.contains(arr[i])) {
                        break;
                    }
 
                    // if element is not present in the
                    // ArrayList then add it to ArrayList
                    // and make ifPresent at true
                    else {
                        al.add(arr[i]);
                        ifPresent = true;
                    }
                }
            }
        }
 
        // if duplicates is present
        // then print ArrayList
        if (ifPresent == true) {
 
            System.out.print(al + " ");
        }
        else {
            System.out.print(
                "No duplicates present in arrays");
        }
    }
 
    // Driver Code
    public static void main(String[] args)
    {
 
        int arr[] = { 12, 11, 40, 12, 5, 6, 5, 12, 11 };
        int n = arr.length;
 
        findDuplicates(arr, n);
    }
}

Output

[12, 11, 5]

Time Complexity: O(N²)
Auxiliary Space: O(N)

Efficient Approach: Use unordered_map for hashing. Count frequency of occurrence of each element and the elements with frequency more than 1 is printed. unordered_map is used as range of integers is not known. For Python, Use Dictionary to store number as key and it’s frequency as value. Dictionary can be used as range of integers is not known.

Below is the implementation of the above approach:

C++

// C++ program to find
// duplicates in the given array
#include <bits/stdc++.h>
using namespace std;
 
// function to find and print duplicates
void printDuplicates(int arr[], int n)
{
    // unordered_map to store frequencies
    unordered_map<int, int> freq;
    for (int i=0; i<n; i++)
        freq[arr[i]]++;
 
    bool dup = false;
    unordered_map<int, int>:: iterator itr;
    for (itr=freq.begin(); itr!=freq.end(); itr++)
    {
        // if frequency is more than 1
        // print the element
        if (itr->second > 1)
        {
            cout << itr->first << " ";
            dup = true;
        }
    }
 
    // no duplicates present
    if (dup == false)
        cout << "-1";
}
 
// Driver program to test above
int main()
{
    int arr[] = {12, 11, 40, 12, 5, 6, 5, 12, 11};
    int n = sizeof(arr) / sizeof(arr[0]);
    printDuplicates(arr, n);
    return 0;
}

Java

// Java program to find
// duplicates in the given array
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
 
public class FindDuplicatedInArray
{
    // Driver program
    public static void main(String[] args)
    {
        int arr[] = {12, 11, 40, 12, 5, 6, 5, 12, 11};
        int n = arr.length;
        printDuplicates(arr, n);
    }
    // function to find and print duplicates
    private static void printDuplicates(int[] arr, int n) 
    {
        Map<Integer,Integer> map = new HashMap<>();
        int count = 0;
        boolean dup = false;
        for(int i = 0; i < n; i++){
            if(map.containsKey(arr[i])){
                count = map.get(arr[i]);
                map.put(arr[i], count + 1);
            }
            else{
                map.put(arr[i], 1);
            }
        }
         
        for(Entry<Integer,Integer> entry : map.entrySet())
        {
            // if frequency is more than 1
            // print the element
            if(entry.getValue() > 1){
                System.out.print(entry.getKey()+ " ");
                dup = true;
            }
        }
        // no duplicates present
        if(!dup){
            System.out.println("-1");
        }
    }
}

Python3

# Python 3 code to find duplicates 
# using dictionary approach.
def printDuplicates(arr):
    dict = {}
 
    for ele in arr:
        try:
            dict[ele] += 1
        except:
            dict[ele] = 1
             
    for item in dict:
         
         # if frequency is more than 1
         # print the element
        if(dict[item] > 1):
            print(item, end=" ")
 
    print("\n")
 
# Driver Code
if __name__ == "__main__":
    list = [12, 11, 40, 12, 
            5, 6, 5, 12, 11]
    printDuplicates(list)
 
# This code is contributed
# by Sushil Bhile

C#

// C# program to find 
// duplicates in the given array 
using System;
using System.Collections.Generic;
 
class GFG
{
    // function to find and print duplicates
    static void printDuplicates(int[] arr, int n)
    {
        Dictionary<int, 
                   int> map = new Dictionary<int, 
                                             int>();
        int count = 0;
        bool dup = false;
        for (int i = 0; i < n; i++)
        {
            if (map.ContainsKey(arr[i]))
            {
                count = map[arr[i]];
                map[arr[i]]++;
            }
            else
                map.Add(arr[i], 1);
        }
 
        foreach (KeyValuePair<int, 
                              int> entry in map)
        {
            // if frequency is more than 1 
            // print the element
            if (entry.Value > 1)
                Console.Write(entry.Key + " ");
            dup = true;
        }
 
        // no duplicates present
        if (!dup)
            Console.WriteLine("-1");
    }
 
    // Driver Code 
    public static void Main(String[] args)
    {
        int[] arr = { 12, 11, 40, 12, 
                     5, 6, 5, 12, 11 };
        int n = arr.Length;
        printDuplicates(arr, n);
    }
}
 
// This code is contributed by
// sanjeev2552

Output

5 12 11

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

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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My Personal Notes

Java

C++

Java

Python3

C#

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