Find first repeating element in an array of integers

To find the first repeating element in an array of integers, you can use a HashSet to keep track of the elements you have encountered so far. Here's a Java implementation:

import java.util.HashSet;

public class FirstRepeatingElement {

    public static int findFirstRepeating(int[] nums) {
        HashSet<Integer> set = new HashSet<>();

        for (int num : nums) {
            if (set.contains(num)) {
                return num;
            }
            set.add(num);
        }

        return -1; // If no repeating element is found
    }

    public static void main(String[] args) {
        int[] nums = {3, 2, 1, 2, 2, 3};
        int firstRepeating = findFirstRepeating(nums);
        if (firstRepeating != -1) {
            System.out.println("First repeating element: " + firstRepeating); // Output: 3
        } else {
            System.out.println("No repeating element found.");
        }
    }
}

In this implementation:

• We use a HashSet to keep track of the elements encountered so far.
• We iterate through the array and check if each element is already present in the HashSet.
• If an element is found to be already present, it is the first repeating element, so we return it.
• If no repeating element is found, we return -1.
• We test the implementation with an example array and print the first repeating element, if found. Otherwise, we print a message indicating that no repeating element is found.