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Find Duplicate Element

In this lesson, you will learn how to find the duplicate element using a hashing algorithm.

Gopi Gorantala
Gopi Gorantala
1 min read

Table of Contents

Problem statement

Given an array of integers A, find one number that repeats. Find the repeated element and return it. If there are no duplicate elements present in the input array, return -1.


Example 01:

Input: A = [3, 4, 1, 4, 2]

Output: 4

Example 02:

Input: A = [-7, 19, 1, 4, 2]

Output: -1

Example 03:

Input: A = [1, 1, 4, 2, 1]

Output: 1 // 1 appeared 3 times in the array

Thought process

Before writing pseudo code, come up with questions and ask the interviewer.

  1. Is the input array sorted?
  2. Should we return -1, when the input array is empty?


  1. Create a HashSet and store the first element.
  2. Iterate through all the elements.
  3. Check if they exist in the HashSet; if yes, return the element. If not, store the element in the HashSet.
  4. Continue the process until the last iteration. If there is no repeating element, return -1.


import java.util.HashSet;

public class FindDuplicateElement {
  public static void main(String[] args) {
    int[] A = {-7, 5, 1, 4, 2, 2};


  private static int findDuplicate(int[] A) {
    HashSet<Integer> uniqueElements = new HashSet<>();

    for (int el : A) {
      if (uniqueElements.contains(el)) {
        return el;

    return -1;

Complexity analysis

Time complexity: O(n), we need to traverse all the elements in the array. If there are n items, then the time complexity to run the algorithm takes O(n).

Space complexity: O(n), this is because we are storing n elements in the HashSet.

Coding Interview QuestionsArraysData Structures and Algorithms

Gopi Gorantala Twitter

Gopi is an engineering leader with 12+ of experience in full-stack development—a specialist in Java technology stack. He worked for multiple startups, the European govt, and FAANG in India and Europe.


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