Given an array of integers, find the first repeating element in it. We need to find the element that occurs more than once and whose index of first occurrence is smallest.

FirstRepeating.java

// http://www.geeksforgeeks.org/find-first-repeating-element-array-integers/
import java.io.*;
import java.util.*;

class FirstRepeating 
{
    void printMajority(int a[], int n) 
    {
        Set<Integer> set = new HashSet<Integer>();
        
        int majority = -1;
        
        for (int i = 0; i < n; i++){
            
            if (set.contains(a[i])) {
                majority = i;
            } else {
                set.add(a[i]);
            }
        }
        
        System.out.println(a[majority]);
    }

    // Driver program to test above functions
    public static void main(String[] args) 
    {
        FirstRepeating obj = new FirstRepeating();
        int arr[] = {10, 5, 3, 4, 3, 5, 6, 10};
        int n = arr.length;
        obj.printMajority(arr, n);
    }
}