How to print repeated elements only once in Java using only array?

Question

The following code snippet works only with duplicated numbers but I'm curious how to solve it with multiple repeated elements.
Input: 1, 2, 5, 5, 5, 8, 9, 11, 12, 1
Output: 1, 5

int[] arr = {1, 2, 5, 5, 5, 8, 9, 11, 12, 1}; 
for (int i = 0; i < arr.length; i++) {
    for (int j = i + 1; j < arr.length; j++) {
        if (arr[i] == arr[j] && i != j) {                  
            System.out.print(arr[j] + " ");
        }
    }
}

@JanezKuhar Well, it would if `forEach` was spelled correctly. — David Conrad, May 08 '20 at 20:00
@HarshalParekh I edited my question, I'm sorry for confusing. — HuserB1989, May 08 '20 at 20:01

score 3 · Accepted Answer · answered May 08 '20 at 20:10

How about this:

int[] arr = { 1, 2, 5, 5, 5, 8, 9, 11, 12, 1 };

boolean[] seen = new boolean[arr.length];

for (int i = 0; i < arr.length; i++)
{
    if(seen[i]) continue;

    boolean duplicate = false;
    for (int j = i + 1; j < arr.length; j++)
    {
        if (arr[i] == arr[j])
        {
            duplicate = seen[j] = true;
        }
    }
    if(duplicate) 
        System.out.print(arr[i] + " ");
}

Output:

1 5

score 1 · Answer 2 · answered May 08 '20 at 20:05

1

int[] arr = {1, 2, 5, 5, 5, 8, 9, 11, 12, 1};

Map<Integer, Integer> map = new HashMap<>();

Arrays.stream(arr).forEach(i -> map.put(i, map.getOrDefault(i, 0) + 1));

map.keySet().stream().mapToInt(i -> i).filter(i -> map.get(i) > 1)
    .forEachOrdered(System.out::println);

You can use a map to store the frequency and then get the numbers with frequency greater than 1.

answered May 08 '20 at 20:05

Harshal Parekh

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1

Thank you for the solution (+1) it is very powerful, however I was curious how to solve it using only arrays. – HuserB1989 May 08 '20 at 20:14
1

This solution or mine is actually better than 2 nested loops. `(O(n) vs O(n^2))`. – Bahij.Mik May 08 '20 at 20:18
You are totally right but I wanted to see how to solve it with using only array as i stretched it in my question :) – HuserB1989 May 08 '20 at 20:25

Bahij.Mik · Answer 3 · 2020-05-08T20:16:15.100

You can do it in one pass using sets (linear time complexity)

public static void getRepeatedValues(int[] values)
    {
        Set<Integer> set = new HashSet<>();
        Set<Integer> ans = new HashSet<>();

        for (int value : values)
        {
            if(set.contains(value))
            {
                ans.add(value);
            } else {
                set.add(value);
            }
        }
        System.out.println(ans);
    }

Input:
[1, 2, 5, 5, 5, 8, 9, 11, 12, 1]
Output:
[1, 5]

score 1 · Answer 4 · answered May 08 '20 at 20:21

You can do it using Stream API as follows:

import java.util.Arrays;
import java.util.Collections;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        Integer[] arr = { 1, 2, 5, 5, 5, 8, 9, 11, 12, 1 };
        Stream.of(arr).distinct().filter(n -> Collections.frequency(Arrays.asList(arr), n) > 1)
                .forEach(System.out::println);
    }
}

Output:

1
5

score 1 · Answer 5 · answered May 08 '20 at 20:37

Here's a simple way to do it with a map.

use the value for the key and the count for the value.
Just count the occurrences and only print the value when the count == 2

int[] arr = { 1, 2, 5, 5, 5, 8, 9, 11, 12, 1 };

Map<Integer, Integer> dups = new HashMap<>();

for (int i : arr) {
    if (dups.compute(i,(k,v)-> v == null ? 1 : v + 1) == 2) {
            System.out.print(i + " ");
    }
}
System.out.println();

Prints

5 1

How to print repeated elements only once in Java using only array?

5 Answers5