function X = sudoku_puzzle(p)
% SUDOKU_PUZZLE  A few Sudoku puzzles.
%   X = sudoku_puzzle(p), for scalar p, returns the p-th puzzle.
%   See the comments in the code for descriptions of the puzzles.

   switch p
      case 1  % Easy, no backtracking required.
         X = [2 0 7 0 9 1 0 0 4; 0 0 0 0 0 0 0 1 2; 6 0 0 0 0 2 5 9 0
              8 0 5 0 2 3 4 0 0; 9 7 0 0 0 0 0 2 6; 0 0 1 7 6 0 9 0 8
              0 8 6 2 0 0 0 0 3; 7 3 0 0 0 0 0 0 0; 5 0 0 6 3 0 1 0 9];

      case 2  % Easy, no backtracking required.
         X = [0 0 4 0 9 0 0 2 1; 0 8 0 0 0 1 5 0 0; 6 9 0 0 0 5 0 0 3
              0 0 0 5 6 3 0 9 4; 0 0 0 0 0 0 0 0 0; 5 2 0 9 4 7 0 0 0
              8 0 0 3 0 0 0 7 2; 0 0 3 2 0 0 0 8 0; 1 6 0 0 7 0 4 0 0];

      case 3  % Slightly difficult, a few backtracking steps.
              % Not unique.
         X = [0 0 0 9 0 0 3 0 5; 0 2 0 0 4 0 0 0 1; 3 0 0 0 8 0 9 0 0
              0 0 0 0 0 3 0 5 0; 0 6 0 4 5 0 8 0 0; 0 0 7 8 0 9 0 0 0
              0 9 2 6 0 4 0 7 0; 6 0 0 0 0 0 0 0 2; 5 7 0 0 0 2 0 3 0];

      case 4  % Moderately difficult, a few hundred backtracing steps.
         X = [7 0 1 0 0 0 4 0 0; 5 0 0 0 0 0 0 0 0; 3 0 0 9 6 0 0 0 0
              0 0 0 3 8 0 0 0 5; 4 7 0 0 0 0 0 0 6; 0 0 0 0 0 9 8 0 2
              0 5 0 0 1 8 0 0 0; 0 2 4 0 0 0 5 0 0; 0 0 0 0 0 3 0 9 0];

      case 5  % From an airline magazine.  Is this a legal puzzle?
         X = [4 0 2 0 0 0 7 0 9; 8 0 5 2 0 4 0 0 0; 0 0 0 0 9 0 0 0 0
              7 0 0 8 0 0 9 0 2; 3 0 0 1 0 5 0 0 8; 1 0 8 0 0 7 0 0 3
              0 0 0 0 3 0 0 0 0; 0 0 0 6 0 1 3 0 7; 9 0 3 0 0 0 6 0 1];

      case 6  % The same airline magazine says this is difficult.  Is it? 
         X = [0 0 0 1 0 0 0 3 0; 0 9 4 3 0 0 7 0 0; 1 0 6 0 0 0 8 2 0
              0 0 0 5 0 0 0 0 0; 6 2 8 0 0 0 5 1 9; 0 0 0 0 0 6 0 0 0
              0 4 1 0 0 0 2 0 5; 0 0 9 0 0 2 4 8 0; 0 8 0 0 0 5 0 0 0];

      case 7  % Solution is not unique.
              % Herzberg & Murty, Notices AMS, v.54, n.6, June 2007.
         X = [9 0 6 0 7 0 4 0 3; 0 0 0 4 0 0 2 0 0; 0 7 0 0 2 3 0 1 0
              5 0 0 0 0 0 1 0 0; 0 4 0 2 0 8 0 6 0; 0 0 3 0 0 0 0 0 5
              0 3 0 7 0 0 0 5 0; 0 0 7 0 0 5 0 0 0; 4 0 5 0 1 0 7 0 8];

      case 8  % Difficult. 
              % Only 17 nonzero initial entries.
              % Herzberg & Murty, Notices AMS, v.54, n.6, June 2007.
         X = [0 0 0 0 0 0 0 1 0; 4 0 0 0 0 0 0 0 0; 0 2 0 0 0 0 0 0 0
              0 0 0 0 5 0 4 0 7; 0 0 8 0 0 0 3 0 0; 0 0 1 0 9 0 0 0 0
              3 0 0 4 0 0 2 0 0; 0 5 0 1 0 0 0 0 0; 0 0 0 8 0 6 0 0 0];

      case 9 % Good example of backtracking.
             % Michael Mephapm, "Diabolical"
             % http://www.sudoku.org.uk/PDF/Solving_Sudoku.pdf 
         X = [0 9 0 7 0 0 8 6 0; 0 3 1 0 0 5 0 2 0; 8 0 6 0 0 0 0 0 0
              0 0 7 0 5 0 0 0 6; 0 0 0 3 0 7 0 0 0; 5 0 0 0 1 0 7 0 0
              0 0 0 0 0 0 1 0 9; 0 2 0 6 0 0 3 5 0; 0 5 4 0 0 8 0 7 0];

      case 10 % "Beware. Very Challenging."
              % Will Shortz, puzzle 301.
              % The Little Black Book of Sudoku.
         X = [0 3 9 5 0 0 0 0 0; 0 0 0 8 0 0 0 7 0; 0 0 0 0 1 0 9 0 4
              1 0 0 4 0 0 0 0 3; 0 0 0 0 0 0 0 0 0; 0 0 7 0 0 0 8 6 0
              0 0 6 7 0 8 2 0 0; 0 1 0 0 9 0 0 0 5; 0 0 0 0 0 1 0 0 8];

      case 11 % "Beware. Very Challenging."
              % Will Shortz, puzzle 400.
              % The Little Black Book of Sudoku.
         X = [0 0 0 0 0 0 8 7 0; 7 8 0 9 4 0 0 0 0; 0 0 0 0 5 0 0 2 0
              0 0 0 3 0 0 0 0 0; 0 5 6 0 0 0 0 0 0; 8 0 9 0 0 2 0 1 0;
              0 0 0 0 9 0 0 8 0; 0 0 0 0 0 0 4 0 7; 0 0 1 7 0 6 0 0 0];

      case 12 % Structural symmetry and not too difficult.
              % American Math Monthly.
              % http://www.maa.org/editorial/mathgames/mathgames_09_05_05.html 
         X = [0 5 0 0 0 0 0 7 0; 9 0 0 6 0 1 0 0 8; 0 0 6 0 2 0 1 0 0;
              0 6 0 0 0 2 0 1 0; 0 0 3 0 0 0 2 0 0; 0 4 0 3 0 0 0 5 0;
              0 0 4 0 3 0 5 0 0; 2 0 0 4 0 5 0 0 9; 0 3 0 0 0 0 0 6 0];

      case 13 % Structural symmetry.
              % Gordon Royle, University of Western Australia.
              % http://people.csse.uwa.edu.au/gordon/sudoku/sudoku-symmetry.pdf
         X = [0 2 0 0 3 0 0 4 0; 6 0 0 0 0 0 0 0 3; 0 0 4 0 0 0 5 0 0
              0 0 0 8 0 6 0 0 0; 8 0 0 0 1 0 0 0 6; 0 0 0 7 0 5 0 0 0
              0 0 7 0 0 0 6 0 0; 4 0 0 0 0 0 0 0 8; 0 3 0 0 4 0 0 2 0];

      case 14 % Very close to matrix symmetry, and very difficult.
              % Gordon Royle, University of Western Australia.
              % http://people.csse.uwa.edu.au/gordon/sudoku/sudoku-symmetry.pdf
         X = [6 0 0 0 0 0 0 0 3; 0 7 0 0 8 0 0 9 0; 0 0 2 0 0 0 5 0 0
              0 0 0 3 0 0 0 0 0; 0 8 0 0 1 0 0 7 0; 0 0 0 0 0 2 0 0 0
              0 0 5 0 0 0 1 0 0; 0 9 0 0 4 0 0 8 0; 3 0 0 0 0 0 0 0 2];

      case 15 % Structural symmetry and only 17 nonzero initial entries.
              % Gordon Royle, University of Western Australia.
              % http://people.csse.uwa.edu.au/gordon/sudoku/sudoku-symmetry.pdf
         X = [0 0 0 0 0 0 0 0 0; 0 0 0 0 0 0 0 1 2; 0 0 3 0 4 5 0 0 0
              0 0 0 6 0 1 0 7 0; 0 0 4 0 0 0 6 0 0; 0 0 5 8 0 0 0 0 0
              0 0 0 0 3 0 4 0 0; 0 1 0 2 0 0 0 0 0; 0 7 0 0 0 0 0 0 0];

      otherwise
         error('sudoku_puzzle(p) expects p in range 1 <= p <= 15')
   end

end % sudoku_puzzles