Initialise an array in spiral form (inside out).

/*

Initialise the center element of the matrix with 1 and then assign all other cells in spiral order.
21 22 23 24 25
26  7   8   9  10
19  6   1   2  11
18  5   4   3  12
17 16 15 14 13

*/

// this will work only for odd number of rows and columns square matrix as in even rows and columns square //matrix there is no center element.

#include<iostream.h>

#include<conio.h>

const int rowcol=5;

int matrix[rowcol][rowcol];

void spiralinvert(int row,int col)

{

     int count=2;

     int cstart=col/2,cend=cstart+1;

     int rstart=row/2,rend=rstart+1;

     int n=row*col;

     matrix[cstart][rstart]=1;

     cout<<"spiral starts"<<endl;

     while(1)

     {

                   

                   for(int i=cstart+1;i<=cend;i++,count++)

                           matrix[rstart][i]=count;

                   cstart--;

                   if(count>=n)

                               break;

                   for(int j=rstart+1;j<=rend;j++,count++)

                                   matrix[j][cend]=count;

                   rstart--;

                   if(count>=n)

                               break;

                   for(int k=cend-1;k>=cstart;k--,count++)

                           matrix[rend][k]=count;

                   cend++;

                   if(count>=n)

                      break;

                   for(int l=rend-1;l>=rstart;l--,count++)

                           matrix[l][cstart]=count;

                   rend++;

                   if(count>=n)

                            break;

     }

     cout<<"spiral ends"<<endl;

}

void display(int row,int col)

{

     for(int i=0;i<row;i++)

     {

             cout<<"\n";

             for(int j=0;j<col;j++)

                     cout<<"\t"<<matrix[i][j];

     }

}

int main()

{

    spiralinvert(rowcol,rowcol);

    display(rowcol,rowcol);

    getch();

    return 0;

}

Print 2D array in spiral form.

/*

if array is 

1    2   3   4   5

6    7   8   9  10

11 12 13 14 15

16 17 18 19 20

21 22 23 24 25

output:- 1 2 3 4 5 10 15 20 25 24 23 22 21 16 11 6 7 8 9 14 19 18 17 12 13

*/

#include<iostream.h>

#include<conio.h>

int matrix[5][5]={1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25};

void spiralprint(int row,int col)

{

     int cstart=0,cend=col-1,rstart=0,rend=row-1,n=col*row;

     int count=0;

     while(count<n)

     {

                   for(int i=cstart;i<=cend;i++,count++)

                           cout<<"  "<<matrix[rstart][i];

                   rstart++;

                   for(int j=rstart;j<=rend;j++,count++)

                           cout<<"  "<<matrix[j][cend];

                   cend--;

                   for(int k=cend;k>=cstart;k--,count++)

                           cout<<"  "<<matrix[rend][k];

                   rend--;

                   for(int l=rend;l>=rstart;l--,count++)

                           cout<<"  "<<matrix[l][cstart];

                   cstart++;

     }

                   

}

int main()

{

    spiralprint(5,5);

    getch();

    return 0;

}

SUDOKU SOLVER

/*

• move from left to right and top to down.
• In the empty cell try 1 to 9 and chose the first one that fit i.e chosen no. should not conflict with any entry in the same row and column.
• In a 9x9 sudoku there are 9 small(3x3) matrix, the chosen number should be unique in this smaller(3x3) matrix.
• Now move on to next cell and repeat the same procedure.
• If no number fits in this cell then backtrack(return false) to the previous made decision and change it to next higher value that fit.
• If no value fits in this cell backtrack to next higher level.

*/

#include<iostream.h>

#include<conio.h>

int sudoku[9][9]={{0,0,4,3,0,2,1,0,0},{0,6,0,0,0,0,0,2,0},{9,0,0,0,0,0,0,0,7},

                  {0,0,1,0,3,0,2,0,0},{2,0,0,9,0,6,0,0,1},{0,0,9,0,5,0,4,0,0},

                  {7,0,0,0,0,0,0,0,3},{0,1,0,0,0,0,0,4,0},{0,0,8,5,0,3,9,0,0}};

                 

bool findunassignedcell(int &r, int &c)

{

     for(int i=0;i<9;i++)

     {

             for(int j=0;j<9;j++)

             {

                     if(sudoku[i][j]==0)

                     {

                                        r=i;

                                        c=j;

                                        return true;

                     }

             }

     }

     return false;

}

void displaysudoku()

{

     cout<<"\n\n";

     for(int i=0;i<9;i++)

     {

              cout<<"\n";

              for(int j=0;j<9;j++)

                      cout<<"  "<<sudoku[i][j];

     }

}

bool noconflict(int r,int c,int n)

{

   bool flag=true;

   for(int i=0;i<9;i++)

   {

           if(sudoku[i][c]==n && i!=r)

                              flag=false;

   }

   for(int j=0;j<9;j++)

   {

           if(sudoku[r][j]==n && j!=c)

                              flag=false;

   }

  

   int rstart=r-r%3,cstart=c-c%3;

   int rend=rstart+2,cend=cstart+2;

  

   for(int p=rstart;p<=rend;p++)

   {

           for(int q=cstart;q<=cend;q++)

           {

                   if(sudoku[p][q]==n && (p!=r && q!=c))

                                      flag=false;

           }

   }

   return flag;

}

            

bool solvesudoku()

{

     int row,col;

     if(!findunassignedcell(row,col))

     {

                                     displaysudoku();

                                     return true;

     }

     for(int num=1;num<=9;num++)

     {

             if(noconflict(row,col,num))

             {

                                      sudoku[row][col]=num;//assign

             }

             else

                 continue;

             if(solvesudoku())

                              return true;

            

              sudoku[row][col]=0;  //unassign

     }

     return false;

}

int main()

{

    cout<<"INTITIAL \n";

    displaysudoku();

    cout<<"\n\nSOLVED \n\n";

    solvesudoku();

    getch();

    return 0;

}