array in first duplicate number code example

Example 1: first duplicate javascript

function firstDuplicate(a) {
    let data = [];
    
    for (dup of a) {
        if (data[dup]) {
            return dup
        } else {
            data[dup] = dup
        }
    }
    return -1
}

Example 2: first duplicate in array

import java.util.*; 
  
class Main 
{ 
    // This function prints the first repeating element in arr[] 
    static void printFirstRepeating(int arr[]) 
    { 
        // Initialize index of first repeating element 
        int min = -1; 
  
        // Creates an empty hashset 
        HashSet<Integer> set = new HashSet<>(); 
  
        // Traverse the input array from right to left 
        for (int i=arr.length-1; i>=0; i--) 
        { 
            // If element is already in hash set, update min 
            if (set.contains(arr[i])) 
                min = i; 
  
            else   // Else add element to hash set 
                set.add(arr[i]); 
        } 
  
        // Print the result 
        if (min != -1) 
          System.out.println("The first repeating element is " + arr[min]); 
        else
          System.out.println("There are no repeating elements"); 
    } 
  
    // Driver method to test above method 
    public static void main (String[] args) throws java.lang.Exception 
    { 
        int arr[] = {10, 5, 3, 4, 3, 5, 6}; 
        printFirstRepeating(arr); 
    } 
}