Arrange eight consecutive number in a matrix so that no two number are adjacent

Question

Need to number 1 to 8 in following diagram such that no two number in neighbor cell are consecutive to each other Pattern :

     *
    ***
    ***
     *

Where each * contain a number between 1 to 8 and no two neighbor * are consucutive number.

Answer 1

Assuming neighbours horizontally and vertically (not diagonally), a greedy solution would be to reshuffle till you find a solution. Your array is small enough, thus you would need a bit more than 24 retries on average to find a match.

import java.util.Random;

public class EightPattern {
    private static Random rnd = new Random();

    /*
     * -0-
     * 123
     * 456
     * -7-
     */
    private static boolean isOK(int[] array) {
        if (       Math.abs(array[0] - array[2]) == 1
                || Math.abs(array[1] - array[2]) == 1
                || Math.abs(array[2] - array[3]) == 1
                || Math.abs(array[1] - array[4]) == 1
                || Math.abs(array[2] - array[5]) == 1
                || Math.abs(array[3] - array[6]) == 1
                || Math.abs(array[4] - array[5]) == 1
                || Math.abs(array[5] - array[6]) == 1
                || Math.abs(array[5] - array[7]) == 1) {
            return false;
        }
        return true;
    }

    //shuffle until you find an isOK solution
    public static void patternShuffle(int[] array) {
        do {
            shuffleArray(array);
        }while(!isOK(array));
    }


    //Fisher–Yates shuffle
    static void shuffleArray(int[] ar) {
        for (int i = ar.length - 1; i > 0; i--) {
            int index = rnd.nextInt(i + 1);
            int a = ar[index];
            ar[index] = ar[i];
            ar[i] = a;
        }
    }

    private static void printPattern(int[] array) {
        System.out.println(" " + array[0]);
        System.out.println("" + array[1] + array[2] + array[3]);
        System.out.println("" + array[4] + array[5] + array[6]);
        System.out.println(" " + array[7]);
    }

    public static void main(String args[]) {
        int[] a = new int[]{1,2,3,4,5,6,7,8};
        patternShuffle(a);
        printPattern(a);
    }   
}