CheckSudokuSolution.java

import java.util.Scanner;

public class CheckSudokuSolution {
  public static void main(String[] args) {
    // Read a Sudoku solution
    int[][] grid = readASolution();

    System.out.println(isValid(grid) ? "Valid solution" 
      : "Invalid solution");
  }

  /** Read a Sudoku solution from the console */
  public static int[][] readASolution() {
    // Create a Scanner
    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;
  }

  /** Check whether a solution is valid */
  public static boolean isValid(int[][] grid) {
    for (int i = 0; i < 9; i++)
      for (int j = 0; j < 9; j++)
        if (grid[i][j] < 1 || grid[i][j] > 9 
            || !isValid(i, j, grid))
          return false;
    return true; // The solution is valid
  }

  /** Check whether grid[i][j] is valid in the grid */
  public static boolean isValid(int i, int j, int[][] grid) {
    // Check whether grid[i][j] is valid at the i's row
    for (int column = 0; column < 9; column++)
      if (column != j && grid[i][column] == grid[i][j])
        return false;

    // Check whether grid[i][j] is valid at the j's column
    for (int row = 0; row < 9; row++)
      if (row != i && grid[row][j] == grid[i][j])
        return false;

    // Check whether grid[i][j] is valid in the 3 by 3 box
    for (int row = (i / 3) * 3; row < (i / 3) * 3 + 3; row++)
      for (int col = (j / 3) * 3; col < (j / 3) * 3 + 3; col++)
        if (row != i && col != j && grid[row][col] == grid[i][j])
          return false;

    return true; // The current value at grid[i][j] is valid
  }
}