Missing Number

/*
 * Problem : Process an array.
 * Input   : n, then n integers.
 * Output  : The missing number.
 * Approach: Compare with the expected sum.
 * 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 long solve(int[] a) {
        // Step 1: The numbers 0..n have the sum n * (n + 1) / 2.
        long n = a.length;
        long expected = n * (n + 1) / 2;

        // Step 2: Add all numbers that are in the array.
        long actual = 0;
        for (int value : a) {
            actual += value;
        }

        // Step 3: The difference is the missing number.
        return expected - actual;
    }

    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();
        }
        System.out.println(solve(a));
    }
}