# The N Queens Problem

## The Problem

The N-Queens problem is a popular puzzle. Assume that you have an NxN chessboard, and N queens. The problem asks you to place all N queens on the board so that no queen can attack another queen. Two queens are attacking (conflicting with) each other if they are on the same row, or same column, or the same diagonal. The diagrams below show examples of invalid positions of two queens that conflict with each other:

* Q * * *        * * * * *        * * * * Q            * * * * *
* * * * *        * Q * * Q        * * * * *            * Q * * *
* * * * *        * * * * *        * * * * *            * * * * *
* Q * * *        * * * * *        * * * * *            * * * Q *
* * * * *        * * * * *        Q * * * *            * * * * *
same            same             same                 same
column           row          minor diagonal       major diagonal


On the other hand, the diagrams below show valid positions of two queens, which do not conflict with each other:


* Q * * *        * * * * *        * * * * Q        * * * * *
* * * * *        * Q * * *        * * * * *        * Q * * *
* * * * *        * * * * Q        * * * * *        * * * * *
* * Q * *        * * * * *        Q * * * *        * * Q * *
* * * * *        * * * * *        * * * * *        * * * * *


For a given N there are usually more than one valid solutions. For example, when N=5, the following solutions are all valid (and there are more):

Q * * * *        * * Q * *        * Q * * *
* * Q * *        * * * * Q        * * * Q *
* * * * Q        * Q * * *        Q * * * *
* Q * * *        * * * Q *        * * Q * *
* * * Q *        Q * * * *        * * * * Q


Perhaps the most famous one is the 8-Queens problem. You can read more about the 8-Queens Problem on Wikipedia

## One Algorithm: Backtracking

There are many algorithms for finding solutions to the N-Queens problem. One of them is a simple backtracking algorithm, which you will implement in this assignment using a stack. The basic idea of the algorithm is to place one queen on each row at a time, but you need to solve for the position of the queen on each row so that no two of them have a column conflict or diagonal conflict.

To do so, you start from the first row, and try to place the queen at each of the N available spots (columns) of that row. However, before placing it, you need to check whether it conflicts with the queens already on the board. Note that when you are working on row k, there should be k-1 queens already placed on the board. If a spot is invalid (i.e. has conflicts with an existing queen in a column or diagonal), it will try the next slot (column); otherwise, if the slot is valid, the algorithm will then record the position of the queen, and continue to work on the next row. What happens if you reach the end of a row but cannot find a valid spot? Well, you backtrack -- moving back to the previous row and trying to place the queen further down in that row.

In a way, this is quite similar to how you solve a maze problem: you start walking in a maze, and when you are at a crossing faced with several turns, you pick one turn and keep walking. At some point you may find that the path you picked is a dead end, so you need to backtrack to the last turn and pick another one, then keep trying until you are out of the maze!

## Implementation

The algorithm can be implemented using recursion. However, In this program you are asked to instead use an explicit stack to keep track of the positions of queens placed so far. Each time the algorithm successfully places a queen in a row, it pushes the column position of that queen onto the stack and starts working on the next row. If it can't find a valid spot, the algorithm backtracks: it returns to the previous row by popping the top element of the stack. This popped element indicates where you were in that previous row. You should then continue searching for valid spots further down in that row. Again, as soon as you find a valid spot, push it to the stack, and work on the next row.

When you find a valid spot on the last row, then congratulations, you've got a solution. At this moment, the positions of queens stored in the stack represent the solution. As an example, look at the 3 example solutions given above for the 5-Queens problem. The stack elements corresponding to the first solution are: 0, 2, 4, 1, 3. Note that the position in each row starts at 0, similar to an array index. Likewise the second solution corresponds to stack elements: 2, 4, 1, 3, 0; and the third solution is: 1, 3, 0, 2, 4.

Your algorithm should find and print out not just one, but all solutions to the N-Queens problem, given some N as input. To find more solutions beyond one, simply continue the searching from the last row.

## Starter code

Starter code is provided. Note that static methods are called directly from main(). If you write any new methods, be sure to make them static as well. The methods of interest are:

• public static int solve(int n) - This method is the primary method called by main() that will search for all solutions to the N-Queens problem given N as a parameter. It returns the number of solutions found. It should also print out each solution by calling the printSolution() method each time a solution is found.

• private static void printSolution(Stack s) - This method prints out a solution in the same manner as the examples provided above (using *'s for empty spaces and Q's for queens). Each element of the stack represents the column position of a queen. This method has been written and should not need to be changed.

The given code uses the Stack class in the java.util package to maintain the search stack. Please check out its API and familiarize yourself with the class.

The only parameter to this program is the number N. By default, N=8, corresponding to the 8-Queens problem. You can pass in a different N by providing it as a command line parameter. For example, the command ‘java NQueens 11’ tells the program that N=11. Alternatively, if you work in Eclipse, you can set the command line parameter by clicking on menu item 'Run', then 'Arguments' tab, and input 11 in the 'Program arguments' edit box.