I was recently just playing around with the n queens problem and decided to try to solve it. I’ve never done it before, but figured that I could use a depth first search kind of solution. I ended up building a solution in node.js which does identify all potential solutions, but decided to put it in angular.js to run on the front end so users can play around and visualize the solutions.

**Essentially, the algorithm works like this:**

- If there’s only 1 queen left to place, and multiple possible positions, then it’s a valid solution. If it’s a unique solution, let’s store it.
- If there is more than one queen left to place, and the number of valid positions left is less than the number of queens, then this is the wrong path, let’s backtrack.
- If neither applies, lets iterate through all the next potential moves in the adjacent column, then recompute the possible positions from that grid state.
- Finally, after we place the queen, we recursively call the algorithm for one less queen, a new grid state, recomputed valid positions and the next column.

**I haven’t had much time to optimize, but a few ideas I could possibly do:**

- Since a chess board is symmetrical, I probably don’t need to check all four corners (since they’ll yield the same result if the board is rotated 90 degrees)
- Use a stack instead for managing the valid positions instead of a simple array. Right now I actually make a copy of all new possible positions after a queen is placed.
- Use a better way to come up with unique solutions than concating a string and comparing each new solution that comes in

**A couple optimizations that already helped:**

- pruning next potential positions to only the starting column + 1 (enormous performance enhancement)
- pruning routes when number of queens left > number of open positions
- cutting down the length of the unique string when comparing objects

In any case, it was a fun little experiment. Here’s the source code, and here’s where you can see it working in jsfiddle. Warning!!! Angular tends to run pretty slow and blocks when you compute, so don’t try to run with too many queens (like > 12 or 13) otherwise you may cause your browser to freeze.

N Queens – http://jsfiddle.net/rocketegg0/wu6cpp5v/

Some average run times:

Num Queens | Solutions | # Comparisons | Run Time (ms) |
---|---|---|---|

4 | 2 | 12 | <1 |

5 | 10 | 41 | 1 |

6 | 4 | 138 | 1 |

7 | 40 | 473 | 1 |

8 | 92 | 1758 | 7 |

9 | 352 | 7077 | 16 |

10 | 724 | 30654 | 45 |

11 | 2680 | 142755 | 360 |

12 | 14200 | 734048 | 4936 |

function Grid(width, height) { this.width = width; this.height = height; var grid = []; var validPositions = []; for (var i = 0; i < width; i++) { grid.push([]); for (var j = 0; j < height; j++) { grid[i][j] = '_'; validPositions.push(new Pair(i,j)) } } this.getValidPositions = function() { return validPositions; } this.getGrid = function() { return grid; } //Position validation function isQueen(pair, x, y) { return pair.x == x && pair.y == y; } function isInRow(pair, y) { return pair.y == y; } function isInCol(pair, x) { return pair.x == x; } //Ex: //3, 2 > 4, 1 = -1, 1 //3, 2 > 4, 3 = -1, -1 //3, 2 > 2, 1 = 1, 1 //3, 2 > 2, 3 = 1, -1 function isInDiagonal(pair, x, y) { if (Math.abs(pair.x - x) == Math.abs(pair.y - y)) { return true; } return false; } function testPosition(pair, x, y) { if (isQueen(pair, x, y) || isInRow(pair, y) || isInCol(pair, x) || isInDiagonal(pair, x, y)) { return true; } return false; } function recomputeValid(validPositions, queenX, queenY) { var newValid = []; for (var i = 0; i < validPositions.length; i++) { if (!testPosition(validPositions[i], queenX, queenY)) { newValid.push(validPositions[i]); } } return newValid; } function gridToString(grid) { var gridstr = ''; for (var i = 0; i < grid.length; i++) { gridstr += '[' for (var j = 0; j < grid[0].length; j++) { gridstr += grid[i][j]; if (j < grid.length-1) { gridstr += '|' } } gridstr += ']' gridstr += '\n'; } return gridstr; } var printGrid = function (grid) { console.log(gridToString(grid)); } var solutions = []; $scope.solution_grids = []; function convertToSolution(grid) { var str = ''; for (var i = 0; i < grid.length; i++) { for (var j = 0; j < grid.length; j++) { if (grid[i][j] == 'Q') { str += i + '#' + j; } } } return str; } this.numcomputations = 0; this.solve = function (numQueens, grid, validPositions, startcol) { if (numQueens <= 1 && validPositions.length > 0) { grid[validPositions[0].x][validPositions[0].y] = 'Q'; var solution = convertToSolution(grid); if (solutions.indexOf(solution) == -1) { solutions.push(solution); $scope.solution_grids.push(gridToString(grid)); } grid[validPositions[0].x][validPositions[0].y] = '_'; //reset return true; } else if (numQueens > validPositions.length) { return false; } else { var x, y; var nextcol = validPositions.filter(function(point) { return point.x == startcol + 1 ? true : false; }); //prune routes to only next col for (var i = 0; i < nextcol.length; i++) { x = nextcol[i].x, y = nextcol[i].y; grid[x][y] = 'Q'; this.solve(numQueens - 1, grid, recomputeValid(validPositions, x, y), startcol+1); grid[x][y] = '_'; //reset this.numcomputations++; } } } } $scope.solve = function() { var nqueens = $scope.sides; var grid = new Grid(nqueens, nqueens); var startTime = new Date().getTime(); console.log('Start Time: ' + startTime); grid.solve(nqueens, grid.getGrid(), grid.getValidPositions(), 0); $scope.numcomputations = grid.numcomputations; $scope.endTime = new Date().getTime() - startTime; console.log('End Time: ' + ($scope.endTime) + 'ms'); };