Java Program to find the first repeating element in array

Simple Java program to find the first repeating element in array

• Let say there is an array
• int array[]={8,7, 5, 3, 4, 3, 5, 6};
• Now if you see the first repeating element is 5
• To solve this, First think comes to our mind is  use iteration.
• then use Set and one counter variable.
• Since we need the first repeating element ,  do the iteration from array.length instead of 0. So that the first repeating element will come at the end in the iteration

public class FirstRepeatingElement {

public static void main(String[] args) throws java.lang.Exception {

int array[] = { 8, 7, 5, 3, 4, 3, 5, 6 };

int min = -1;

// Creates an empty hashset

HashSet<Integer> set = new HashSet<>();

// Traverse the input array from right to left

for (int i = array.length - 1; i >= 0; i--) {

// If element is already in hash set, update min

if (set.contains(array[i])) {

min = i;

}

else {

// Else add element to hash set

set.add(array[i]);

}

}

if (min != -1) {

System.out.println("The first repeating element is " + array[min]);

} else {

System.out.println("There are no repeating elements");

}

}

}

• As you can see, Hashset will not allow duplicate, whenever it finds the duplicate element just increment the counter min variable.
• And also it iterate the for loop in reverse direction.

Alternate approach is Use LinkedHashMap.

• Initialize LinkedHashMap<Integer, Integer> hMap=new LinkedHashMap<>();
• Iterate the array element in for loop
• Check the map contains the same element already
• if not put the element in hMap with value 1
• if already found, then just increment the value to +1

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

if(hMap.get(arr[i]) == null) {

hMap.put(arr[i], 1);

}else {

int val=hMap.get(arr[i]);

hMap.put(arr[i],val+1);

}

}

Finally, iterate the Map, the print the first repeating element in array.