Subarray with a Given Sum (Positive Numbers)

/*
 * Problem : Subarray with sum k.
 * Input   : n, the array, then k.
 * Output  : Start and end or -1.
 * Approach: Sliding window.
 * Time    : O(n)
 * Space   : O(1)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
    // This method solves the task. It receives the input and returns the answer.
    static String solve(int[] a, int k) {
        // Step 1: The window is a[left..right]. "sum" is the sum of the window.
        long sum = 0;
        int left = 0;
        for (int right = 0; right < a.length; right++) {
            sum += a[right];
            // Step 2: Too big: shrink the window from the left. All numbers are positive, so this only lowers the sum.
            while (sum > k && left <= right) {
                sum -= a[left++];
            }
            // Step 3: The first window that reaches exactly k is the answer.
            if (sum == k) {
                return (left + 1) + " " + (right + 1);
            }
        }
        return "-1";
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int size = scanner.nextInt();
        int[] a = new int[size];
        for (int i = 0; i < size; i++) {
            a[i] = scanner.nextInt();
        }
        int k = scanner.nextInt();
        System.out.println(solve(a, k));
    }
}