### Find First Duplicate in an Array Java - The Coding Shala

Home >> Interview Questions >> first duplicate element In this post, we will discuss how to find first duplicate in an array and will implement its solution in Java. First Duplicate in an Array Problem Given an array a that contains only numbers in the range from 1 to a.length, find the first duplicate number for which the second occurrence has the minimal index. In other words, if there are more than 1 duplicated numbers, return the number for which the second occurrence has a smaller index than the second occurrence of the other number does. If there are no such elements, return -1. Example 1: For a = [2, 1, 3, 5, 3, 2], the output should be firstDuplicate(a) = 3. There are 2 duplicates: numbers 2 and 3. The second occurrence of 3 has a smaller index than the second occurrence of 2 does, so the answer is 3. First Duplicate in an Array Java Solution Approach 1: We can use HashSet. If a number repeat, we will return that. It will take an additional O(n) space to sto