Problem011.java

/**
 * Problem 11:
 * What is the greatest product of four adjacent numbers in the same direction
 * (up, down, left, right, or diagonally) in the 20×20 grid?
 */
public class Problem011 {

    private static int[][] grid = new int[][] {
                    {8, 2, 22, 97, 38, 15, 0, 40, 0, 75, 4, 5, 7, 78, 52, 12, 50, 77, 91, 8},
                    {49, 49, 99, 40, 17, 81, 18, 57, 60, 87, 17, 40, 98, 43, 69, 48, 4, 56, 62, 0},
                    {81, 49, 31, 73, 55, 79, 14, 29, 93, 71, 40, 67, 53, 88, 30, 3, 49, 13, 36, 65},
                    {52, 70, 95, 23, 4, 60, 11, 42, 69, 24, 68, 56, 1, 32, 56, 71, 37, 2, 36, 91},
                    {22, 31, 16, 71, 51, 67, 63, 89, 41, 92, 36, 54, 22, 40, 40, 28, 66, 33, 13, 80},
                    {24, 47, 32, 60, 99, 3, 45, 2, 44, 75, 33, 53, 78, 36, 84, 20, 35, 17, 12, 50},
                    {32, 98, 81, 28, 64, 23, 67, 10, 26, 38, 40, 67, 59, 54, 70, 66, 18, 38, 64, 70},
                    {67, 26, 20, 68, 2, 62, 12, 20, 95, 63, 94, 39, 63, 8, 40, 91, 66, 49, 94, 21},
                    {24, 55, 58, 5, 66, 73, 99, 26, 97, 17, 78, 78, 96, 83, 14, 88, 34, 89, 63, 72},
                    {21, 36, 23, 9, 75, 0, 76, 44, 20, 45, 35, 14, 0, 61, 33, 97, 34, 31, 33, 95},
                    {78, 17, 53, 28, 22, 75, 31, 67, 15, 94, 3, 80, 4, 62, 16, 14, 9, 53, 56, 92},
                    {16, 39, 5, 42, 96, 35, 31, 47, 55, 58, 88, 24, 0, 17, 54, 24, 36, 29, 85, 57},
                    {86, 56, 0, 48, 35, 71, 89, 7, 5, 44, 44, 37, 44, 60, 21, 58, 51, 54, 17, 58},
                    {19, 80, 81, 68, 5, 94, 47, 69, 28, 73, 92, 13, 86, 52, 17, 77, 4, 89, 55, 40},
                    {4, 52, 8, 83, 97, 35, 99, 16, 7, 97, 57, 32, 16, 26, 26, 79, 33, 27, 98, 66},
                    {88, 36, 68, 87, 57, 62, 20, 72, 3, 46, 33, 67, 46, 55, 12, 32, 63, 93, 53, 69},
                    {4, 42, 16, 73, 38, 25, 39, 11, 24, 94, 72, 18, 8, 46, 29, 32, 40, 62, 76, 36},
                    {20, 69, 36, 41, 72, 30, 23, 88, 34, 62, 99, 69, 82, 67, 59, 85, 74, 4, 36, 16},
                    {20, 73, 35, 29, 78, 31, 90, 1, 74, 31, 49, 71, 48, 86, 81, 16, 23, 57, 5, 54},
                    {1, 70, 54, 71, 83, 51, 54, 69, 16, 92, 33, 48, 61, 43, 52, 1, 89, 19, 67, 48}};

    private static int greatestProduct() {
        int greatest = 0;
        int p;
        for (int i=0; i<20; i++) {
            for (int j=0; j<20; j++) {
                // Down
                if (i < 17) {
                    p = grid[i][j] * grid[i+1][j] * grid[i+2][j] * grid[i+3][j];
                    if (p > greatest)
                        greatest = p;
                }
                // Right
                if (j < 17) {
                    p = grid[i][j] * grid[i][j+1] * grid[i][j+2] * grid[i][j+3];
                    if (p > greatest)
                        greatest = p;
                }
                // Diagonally
                if (i < 17 && j < 17) {
                    p = grid[i][j] * grid[i+1][j+1] * grid[i+2][j+2] * grid[i+3][j+3];
                    if (p > greatest)
                        greatest = p;
                }
                if (i > 2 && j < 17) {
                    p = grid[i][j] * grid[i-1][j+1] * grid[i-2][j+2] * grid[i-3][j+3];
                    if (p > greatest)
                        greatest = p;
                }
            }
        }
        return greatest;
    }

    public static void main(String[] args) {
        System.out.println("The greatest product of four adjacent numbers in the same direction is:");
        System.out.println(greatestProduct());
    }

}