Row Sums and Column Sums

/*
 * Problem : Process a matrix.
 * Input   : r and c, then r rows of c integers.
 * Output  : Two lines.
 * Approach: One pass for both sums.
 * Time    : O(r * c)
 * Space   : O(r + c)
 * 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[][] m) {
        int rows = m.length;
        int cols = m[0].length;

        // Step 1: One sum for each row and one sum for each column.
        long[] rowSums = new long[rows];
        long[] colSums = new long[cols];

        // Step 2: Add every element to its row sum and to its column sum.
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                rowSums[i] += m[i][j];
                colSums[j] += m[i][j];
            }
        }

        // Step 3: Make the two output lines.
        StringBuilder first = new StringBuilder("rows:");
        for (long sum : rowSums) {
            first.append(" ").append(sum);
        }
        StringBuilder second = new StringBuilder("columns:");
        for (long sum : colSums) {
            second.append(" ").append(sum);
        }
        return new String[] {first.toString(), second.toString()};
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int rows = scanner.nextInt();
        int cols = scanner.nextInt();
        int[][] m = new int[rows][cols];
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                m[i][j] = scanner.nextInt();
            }
        }
        System.out.println(String.join("\n", solve(m)));
    }
}