Array elements that appear more than once

Given an integer array, print all repeating elements (Elements that appear more than once) in the array. The output should contain elements according to their first occurrences.

Examples:

Input: arr[] = {12, 10, 9, 45, 2, 10, 10, 45}
Output: 10 45

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

Input: arr[] = {1, 1, 1, 1, 1}
Output: 1

The idea is to use Hashing to solve this in O(n) time on average. We store elements and their counts in a hash table. After storing counts, we traverse input array again and print those elements whose counts are more than once. To make sure that every output element is printed only once, we set count as 0 after printing the element.

C++

// C++ program to print all repeating elements
#include <bits/stdc++.h>
using namespace std;

void printRepeating(int arr[], int n)
{
    // Store elements and their counts in
    // hash table
    unordered_map<int, int> mp;
    for (int i = 0; i < n; i++)
        mp[arr[i]]++;

    // Since we want elements in same order,
    // we traverse array again and print
    // those elements that appear more than
    // once.
    for (int i = 0; i < n; i++) {
        if (mp[arr[i]] > 1) {
            cout << arr[i] << " ";

            // This is tricky, this is done
            // to make sure that the current
            // element is not printed again
            mp[arr[i]] = 0;
        }
    }
}

// Driver code
int main()
{
    int arr[] = { 12, 10, 9, 45, 2, 10, 10, 45 };
    int n = sizeof(arr) / sizeof(arr[0]);
    printRepeating(arr, n);
    return 0;
}

Java

// Java program to print all repeating elements

import java.util.*;
import java.util.Map.Entry;
import java.io.*;
import java.lang.*;

public class GFG {

    static void printRepeating(int arr[], int n)
    {

        // Store elements and their counts in
        // hash table
        Map<Integer, Integer> map
            = new LinkedHashMap<Integer, Integer>();
        for (int i = 0; i < n; i++) {
            try {
                map.put(arr[i], map.get(arr[i]) + 1);
            }
            catch (Exception e) {
                map.put(arr[i], 1);
            }
        }

        // Since we want elements in the same order,
        // we traverse array again and print
        // those elements that appear more than once.

        for (Entry<Integer, Integer> e : map.entrySet()) {
            if (e.getValue() > 1) {
                System.out.print(e.getKey() + " ");
            }
        }
    }

    // Driver code
    public static void main(String[] args) throws IOException
    {
        int arr[] = { 12, 10, 9, 45, 2, 10, 10, 45 };
        int n = arr.length;
        printRepeating(arr, n);
    }
}

// This code is contributed by Wrick

Python3

# Python3 program to print
# all repeating elements
def printRepeating(arr, n):

    # Store elements and
    # their counts in
    # hash table
    mp = [0] * 100
    for i in range(0, n):
        mp[arr[i]] += 1

    # Since we want elements
    # in same order, we
    # traverse array again
    # and print those elements
    # that appear more than once.
    for i in range(0, n):
        if (mp[arr[i]] > 1):
            print(arr[i], end = " ")

            # This is tricky, this
            # is done to make sure
            # that the current element
            # is not printed again
            mp[arr[i]] = 0

# Driver code
arr = [12, 10, 9, 45,
       2, 10, 10, 45]
n = len(arr)
printRepeating(arr, n)

# This code is contributed
# by Smita

C#

// C# program to print all repeating elements
using System;
using System.Collections.Generic;

class GFG
{
static void printRepeating(int []arr, int n)
{

    // Store elements and their counts in
    // hash table
    Dictionary<int,
               int> map = new Dictionary<int,
                                         int>();
    for (int i = 0 ; i < n; i++)
    {
        if(map.ContainsKey(arr[i]))
        {
            var val = map[arr[i]];
            map.Remove(arr[i]);
            map.Add(arr[i], val + 1);
        }
        else
        {
            map.Add(arr[i], 1);
        }
    }

    // Since we want elements in the same order,
    // we traverse array again and print
    // those elements that appear more than once.
    foreach(KeyValuePair<int, int> e in map)
    {
        if (e.Value > 1)
        {
            Console.Write(e.Key + " ");
        }
    }
}

// Driver code
public static void Main(String[] args)
{
    int []arr = { 12, 10, 9, 45, 2, 10, 10, 45 };
    int n = arr.Length;
    printRepeating(arr, n);
}
}

// This code is contributed by PrinciRaj1992

Output:

10 45

Time Complexity: O(n) under the assumption that hash insert and search functions work in O(1) time.

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