Java · JavaScript · Programming

Interesting Algorithm Questions for interview

Find the duplicates in an array in O(n) time

There is actually no algorithm to give perfect O(n) time for processing.
However, there is a way to find the solution in worst case scenario of O(2n) i.e O(n).

ke’s comment about the complexity being O(n2) is indeed correct.
You can read more about it at post.

I present the solution below in JavaScript and Java.
It is a simple solution based on Associative Arrays in JavaScript and Hash Maps in Java.
In each case you store the elements of input array into a hash based datastructure.
This ensures that you do not repeat the same element and can count the number of instances of duplicate elements.

JavaScript solution

//find the duplicates in an array in O(n)
function findDupsInArray() {
    var arr = new Array("1","1","2","3","1","3","4","4","5","5","5","5","23","55","23");

    var assArr = new Array();

        if(arr[i] in assArr) 
            assArr[arr[i]] = assArr[arr[i]]+1;
            assArr[arr[i]] = 1;

    for(var key in assArr) {
        alert(key + ' = ' + assArr[key]);



Java Solution

package algorithms;

import java.util.Map;
import java.util.HashMap;

 * @author amitrai
public class FindDuplicatesInArray {

    public static void main(String[] args) {

        String[] arr = {"1","1","2","3","1","3","4","4","5","5","5","5","23","55","23"};

        Map dataMap = new HashMap();

        for(int i=0;i<arr.length;i++) {

            if(dataMap.containsKey(arr[i])) {
                dataMap.put(arr[i], dataMap.get(arr[i])+1);
            } else {
                dataMap.put(arr[i], 1);


        for(String key : dataMap.keySet()) {



2 thoughts on “Interesting Algorithm Questions for interview

  1. Not correct. First, there is no difference between O(n) and O(2n). Second, containsKey itself has O(n) worst case time so you get O(n²) overall.

    Credits: Thanks to D. and Y. for confirming my hunch on this.

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