Flood Fill: Region Size

/*
 * Problem : Size of the region.
 * Input   : r c sr sc, then the rows.
 * Output  : The number of cells.
 * Approach: Recursive fill
 * Time    : O(r c)
 * Space   : O(r c)
 * Java    : 8 or newer.
 */
import java.util.Scanner;

public class Main {
        // Step 2: Visit one cell. A cell outside the grid, a different value, or a cell that was visited before stops the recursion.
        static int fill(char[][] grid, boolean[][] seen, int row, int col, char value) {
            if (row < 0 || col < 0 || row >= grid.length || col >= grid[0].length || seen[row][col] || grid[row][col] != value) {
                return 0;
            }
            seen[row][col] = true;
            // Step 3: This cell counts 1, plus everything that can be reached from its four neighbours.
            return 1 + fill(grid, seen, row + 1, col, value) + fill(grid, seen, row - 1, col, value) + fill(grid, seen, row, col + 1, value) + fill(grid, seen, row, col - 1, value);
        }

    // This method solves the task. It receives the input and returns the answer.
    static int solve(String[] lines) {
        // Step 1: Read the size, the start cell and the rows.
        String[] head = lines[0].split(" ");
        int rows = Integer.parseInt(head[0]);
        int cols = Integer.parseInt(head[1]);
        int startRow = Integer.parseInt(head[2]);
        int startCol = Integer.parseInt(head[3]);
        char[][] grid = new char[rows][];
        for (int i = 0; i < rows; i++) {
            grid[i] = lines[i + 1].toCharArray();
        }
        return fill(grid, new boolean[rows][cols], startRow, startCol, grid[startRow][startCol]);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        java.util.List<String> all = new java.util.ArrayList<>();
        while (scanner.hasNextLine()) {
            String line = scanner.nextLine().trim();
            if (!line.isEmpty()) {
                all.add(line);
            }
        }
        String[] lines = all.toArray(new String[0]);
        System.out.println(solve(lines));
    }
}