NQueen Problem using backtracking.

File: Location.h

#ifndef nQueen_Location_h
#define nQueen_Location_h

const int UNDEFINED = -1;

class Location {
private:
    int row;
    int col;
public:
    Location();
    Location( int row, int col );
    void setLocation( int row, int col );
    void setLocation( Location* location );
    int getRow();
    int getCol();
};
#endif

File: Location.cpp

#include "Location.h"

Location::Location(){
    row = UNDEFINED;
    col = UNDEFINED;
};

Location::Location( int _row, int _col ){
    row = _row;
    col = _col;
};

void Location::setLocation( int _row, int _col ) {
    row = _row;
    col = _col;
};
void Location::setLocation( Location* location ) {
    row = location->getRow();
    col = location->getCol();
};

int Location::getCol() {
    return col;
};

int Location::getRow(){
    return row;
};

File: Queen.h

#ifndef nQueen_Queen_h
#define nQueen_Queen_h

#include "Location.h"
#include <stdlib.h>

class Queen {
private:
    Location* location;
    
public:
    Queen();
    Queen( int row, int col );
    bool canAttack( int row, int col );
    int getRow();
    int getCol();
    Location* getLocation();
    void setLocation(int row, int col);
    void resetLocation();
    ~Queen();   
};

#endif

File: Queen.cpp

#include "Queen.h"
Queen::Queen() {
    Queen( UNDEFINED, UNDEFINED );
};

Queen::Queen( int row, int col ){
    location = new Location( row, col );
};

bool Queen::canAttack( int row, int col ){
    int myRow = getRow();
    int myCol = getCol();
    if ( myCol == UNDEFINED || myRow == UNDEFINED ) {
        return false;
    }
    if ( myRow == row || myCol == col ) {
        return true;
    }
    /* Compute the all 4 diagonal */
    int rowDiff = abs ( row - myRow );
    int colDiff = abs ( col - myCol );
    if ( rowDiff == colDiff ) {
        return true;
    }
    return false;
};

Location* Queen::getLocation() {
    return location;
};

void Queen::setLocation( int _row, int _col ){
    
    if ( location == nullptr ) {
        location = new Location( _row, _col);
        return;
    }
    location->setLocation(_row, _col );
   
};

void Queen::resetLocation() {
    setLocation(UNDEFINED, UNDEFINED);
}

int Queen::getRow() {
    return location->getRow();
}

int Queen::getCol() {
    return location->getCol();
}

Queen::~Queen(){
    delete location;
}

File: Board.h

#ifndef nQueen_Board_h
#define nQueen_Board_h

#include "Queen.h"

class Board{
private:
    Queen* queens;
    int boardSize;
    int numOfSoln;
    bool _solve( int row, Queen* queen );
    bool isCellSafe( int row, int col );
public:
    Board( int boardSize);
    void print();
    void solve();

};
#endif

File: Board.cpp

#include "Board.h"
#include<iostream>
using namespace std;

bool Board::_solve( int row, Queen* queen ){
    
    if ( row == boardSize ) {
        print();
        return true;
    }
    
    Queen* subQueen = queen;
    for ( int col = 0; col < boardSize; col ++ ) {
        if ( isCellSafe(row, col ) ) {
            subQueen->setLocation( row, col );
            _solve(row + 1, subQueen + 1 );
            subQueen->resetLocation();
        }
    }
    return false;
    
};

bool Board::isCellSafe( int row, int col ) {
    Queen* tmpQueen = queens;
    for ( int i = 0 ; i < boardSize; i ++ ) {
        if ( tmpQueen->canAttack( row, col ) ) {
            return false;
        }
        tmpQueen++;
    }
    return true;
};

Board::Board( int _boardSize) {
    
    boardSize = _boardSize;
    queens = new Queen[boardSize];
    for( int i = 0 ; i < boardSize; i++ ) {
        Queen* tmpQueen = new Queen( UNDEFINED, UNDEFINED );
        queens[i] = *tmpQueen;
    }
};

void Board::print(){
    Queen* rowQueen = queens;
    numOfSoln++;
    cout << "Printing solution: " << numOfSoln << "\n";
    for( int rownum = 0 ; rownum < boardSize; rownum++ ) {
        cout << rownum << ", " << rowQueen->getCol() << endl;
        rowQueen++;
    }
};

void Board::solve() {
    if ( _solve(0, queens ) ){
        print();
        return;
    }
    if ( numOfSoln == 0 ) {
        cout << "Could not find a solution" << endl;
    }   
};

File: main.cpp

#include<iostream>

#include "Board.h"

int main(int argc, const char * argv[]) {
    
    if ( argc !=  2 ) {
        fprintf(stderr, "Please provide board size.");
        return 1;
    }
    int size = atoi( argv[1]);
    Board* board = new Board( size);
    board->solve();
    delete board;
    return 0;
}

If something is not write please let me know.

Password Less SSH

SSH is linux utility to logon to a remote machine for executing command on it.
Every time you ssh to any machine you need to provide username/password to securely login on that machine.

Once a username/password is authenticated, you are logged on that machine and can execute the commands on the remote machine. But to successfully execute a password on remote machine without human intervention, you need to setup SSH such that it don't prompt for password.
E.g.: Hadoop applications requires that Master machine should be able to login to all the slaves machine in cluster password less SSH.

 Linux SSH utility provides a easy way to achieve the password less SSH.

Create  SSH Keys
     ssh-keygen -t rsa  -f ~/.ssh/id_rsa -P ''
       The options are
         -t rsa: Specifies the type of key to create
         -f Specifies the filename of the key file.
         -P Passpharse. Note: we are using empty passphrase.

Copy the public key on the remote machine.
    scp ~/.ssh/id_rsa.pub user@remote.machine:/tmp/id_rsa_.pub_test

Log on to the remote.machine.

Append the content of the file to .ssh/authtozed_keys.
     cat /tmp/id_rsa_.pub_test > ~/.ssh/authorized_keys

Ensure following on the remote machine.
   -  ~/.ssh has permission set to 700
   -  ~/.ssh/authorized_keys has permission set to 640

Alternatively you could use ssh-copy-id command to copy the public key on the remote machine. ssh-copy-id ensures that the correct permissions are set on the remote machine.

  > ssh-copy-id user@remote.machine

Try to SSH on the remote machine, SSH should not ask for the passphase and you should be logged on the remote server.

Reference:
    ssh-keygen linux man page.
    ssh-copy-id linux man page.