Unique Paths with Obstacles

/*
 * Problem : Count robot paths.
 * Input   : r and c, then the grid.
 * Output  : Number of paths.
 * Approach: One row of counts.
 * Time    : O(r * c)
 * Space   : O(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 long solve(int[][] m) {
        // Step 1: ways[j] is the number of paths to the cell in column j of the current row. We keep one row only.
        int columns = m[0].length;
        long[] ways = new long[columns];
        ways[0] = 1;
        for (int[] row : m) {
            for (int j = 0; j < columns; j++) {
                // Step 2: A blocked cell has no paths. Otherwise add the paths from the left cell. The cell above is already in ways[j].
                if (row[j] == 1) {
                    ways[j] = 0;
                } else if (j > 0) {
                    ways[j] += ways[j - 1];
                }
            }
        }
        return ways[columns - 1];
    }

    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(solve(m));
    }
}