# Write a backtracking algorithm for the n-queens problem

The Nonattacking Chess Queens Problem The nonattacking chess queens problem consists of finding positions for n queens on an nxn square board so that no two of the queens are attacking one another according to the usual rules of chess. Specifically, no two queens can share a row, a column, or a diagonal. This problem is also known simply as the n queens problem. Some collections of these choices will result in a failed task, while other collections result in an accomplished task. If one simply searches through all possible collections of choices looking for which ones are successful i. As an example of this, consider the N-queens problem: Place N queens on an N x N chessboard so that no two queens attack each other -- that is to say, no two queens share the same row, column, or diagnonal.

We need a better way One such better way is through the use of a backtracking algorithm.

 N Queens problem LISP (ASSIGNMENT) | Algorithm | Lisp Backtracking We assume that a solution to the problem can be constructed from component solutions, xi. java - N Queens program using stack and backtracking - Stack Overflow Whether your application is business, how-to, education, medicine, school, church, sales, marketing, online training or just for fun, PowerShow. And, best of all, most of its cool features are free and easy to use. Developer Shed Affiliates Source I ran across an interesting problem recently while studying some recursive algorithms: That is, for a valid configuration solution no two queens share the same row rankcolumn fileor diagonal.

In a backtracking algorithm, one incrementally builds candidates to the solution s and abandons partial candidates backtracks as soon as it is determined it cannot possibly be a valid solution. For the N-Queens problem, one way we can do this is given by the following: For each row, place a queen in the first valid position columnand then move to the next row If there is no valid position, then one backtracks to the previous row and try the next position If one can successfully place a queen in the last row, then a solution is found.

Now backtrack to find the next solution Using a Stack for Backtracking in the N-Queens Problem Recall that each queen must be on a different row in the N-Queens problem. Given this, we shall attempt to put queens on the board one row at a time starting with row 0. We can use a stack to indicate the positions of the queens. Importantly, notice that we only have to put the column positions of the queens on the stack. We simply combine the position of an element in the stack the row with the value of that element the column for each queen.

Two examples of this are shown below: If we find a valid position in this row, we push this position i. Of course, there is nothing that requires there be only one solution.

To find the rest, every time a solution is found, we can pretend it is not a solution, backtrack to the previous row, and proceed to find the next solution.

## Hamiltonian Circuit Problem

Ultimately, every position in the first row will be considered. Putting all this into pseudo-code form, we have the following algorithm If you actually want to implement the above algorithm, having a method that checks to see if a position is valid given the queens placed so far might be a really good idea.

This of course means you will have to check the position against all previous queen positions in previous rows all of which can be determined from the stack.7 BACKTRACKING: Solution Space Tuples that satisfy the explicit constraints define a solution space.

The solution space can be organized into a tree. Each node in the tree defines a problem state. All paths from the root to other nodes define the state-space of the problem. Algorithms monstermanfilm.com Backtracking: Largest maximal independent set A simple example of average-case analysis A simple example of a backtracking algorithm is the N-Queens problem.

The interactive applet on this page demonstrates how a computer can solve the N by N queens problem. It also can be used to show all solutions for N=4,5,6,7,8, and to . If the interviewer had asked you to write the code for the problem, then I think that it is not fair.

The algorithm requires work. However, if the idea was to show the interviewer the classes, methods or some concepts that you'd need to use or something similar, then it may be a fair question.

Backtracking 3 – For both the above formulations, the solution space is 2n distinct tuples n-queen problem – A generalization of the 8-queen problem – Place n queens on an n n chessboard so that no queen attacks another queen. CS, Machine Learning Prof.

Alvarez GA Example: The Nonattacking Chess Queens Problem The nonattacking chess queens problem consists of finding positions for n queens on an nxn square board so that no two of the queens are attacking one another according to the usual rules of chess.

N Queens Problem in C Using Backtracking - The Crazy Programmer