Subarray Sum Equals K

/*
 * Problem : Process an array and a number k.
 * Input   : n, integers, then k.
 * Output  : The number of subarrays.
 * Approach: Prefix sums and a map.
 * Time    : O(n)
 * Space   : O(n)
 * 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 int solve(int[] a, int k) {
        // Step 1: seen counts how many earlier prefix sums have each value. The empty prefix has the sum 0.
        java.util.Map<Long, Integer> seen = new java.util.HashMap<>();
        seen.put(0L, 1);

        long prefix = 0;
        int count = 0;

        for (int value : a) {
            // Step 2: The prefix sum up to this element.
            prefix += value;

            // Step 3: A subarray ending here has the sum k if some earlier prefix sum is prefix - k.
            count += seen.getOrDefault(prefix - k, 0);

            // Step 4: Remember this prefix sum.
            seen.put(prefix, seen.getOrDefault(prefix, 0) + 1);
        }
        return count;
    }

    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));
    }
}