Sudoku.java

import java.util.Scanner;
public class Sudoku
{ 
    public static void main (String[] args) 
    { 
        int[][] grid = readASolution();
        System.out.println(isValid(grid) ? "Valid solution" : "Invalid solution");
    }

    public static int[][] readASolution() {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter a sudoku puzzle solution");
        int[][] grid = new int[9][9];
        for(int i=0; i<9; i++) {
            for(int j=0; j<9; j++) {
                grid[i][j] = input.nextInt();
            }
        }
        return grid;
    }

    public static boolean isValid(int[][] grid){
        //check rows
        for(int i = 0; i < 9; i++) {
            if (!is1To9(grid[i])){
                return false;
            }
        }

        //check columns
        for (int j=0; j<9; j++) {
            int[] column = new int[9];
            for(int i=0; i<9; i++) {
                column[i] = grid[i][j];
            }
            if (!is1To9(column)){
                return false;
            }
        }

        //check cells
        for (int i=0; i<3; i++){
            for (int j=0; j<3; j++) {
                //starting element in the 3x3 box
                int k = 0;
                int[] list = new int[9];
                for(int row = i * 3; row < i* 3 + 3; row++) {
                    for (int column = j * 3; column < j * 3 + 3; column++) {
                        list[k++] = grid[row][column];
                    }
                }
                if (!is1To9(list)) {
                    return false;
                }
            }            
        }
        return true;
    }

    public static boolean is1To9(int[] list) {
        int[] temp = new int[list.length];
        System.arraycopy(list, 0, temp, 0, list.length);
        java.util.arrays.sort(temp);
        for(int i = 0; i < 9; i++) {
            if (temp[i] != i+1) {
                return false;
            }
            return true;
        }    
    }
}