Add OS X/Swift and Python implementations of the Eight Queens problem

EightQueens-OSX-Swift/EightQueens/Queen.swift

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+//
+//  Queen.swift
+//  EightQueens
+//
+//  Created by Donald Burr on 5/27/15.
+//  Copyright (c) 2015 Donald Burr. All rights reserved.
+//
+
+import Foundation
+
+class Queen {
+    var row: Int
+    var column: Int
+    
+    init(forRow: Int) {
+        row = forRow
+        column = 0
+    }
+}

EightQueens-OSX-Swift/EightQueens/main.swift

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+//
+//  main.swift
+//  EightQueens
+//
+//  Created by Donald Burr on 5/27/15.
+//  Copyright (c) 2015 Donald Burr. All rights reserved.
+//
+
+import Foundation
+
+var solutionsFound = 0
+var positionsChecked = 0
+
+// create 8 queen objects, place them one per row
+let myQueens = map(0...7) { Queen(forRow: $0) }
+
+func isSafe(currentRow: Int, currentColumn: Int) -> Bool {
+    positionsChecked++
+    
+    // iterate over all queens in previous rows
+    for previousRow in 0..<currentRow {
+        // check vertical: are we trying to place in the same column as a previously placed queen?
+        if myQueens[previousRow].column == currentColumn {
+            return false
+        }
+        // check diagonal
+        let differenceInRows = currentRow - previousRow
+        let differenceInColumns = currentColumn - myQueens[previousRow].column
+        if (differenceInRows == differenceInColumns || differenceInRows == -differenceInColumns) {
+            return false
+        }
+    }
+    
+    // compared all previous queens, no conflict
+    myQueens[currentRow].column = currentColumn
+    return true
+}
+
+func moveQueenAcrossRow(row: Int) {
+    for column in 0...7 {
+        // move queen column by column, checking if it's safe
+        if isSafe(row, column) {
+            // if we just placed the 8th queen, we have a result!
+            if row == 7 {
+                solutionsFound++
+                printAnswer()
+            } else {
+                // recursive call - now move the queen to the NEXT row
+                moveQueenAcrossRow(row+1)
+            }
+        }
+    }
+}
+
+func printAnswer() {
+    println("SOLUTION #\(solutionsFound)")
+    println()
+    
+    // go backwards to print from top down
+    for currentRow in reverse(0...7) {
+        // print row number (using chess numbering)
+        print(currentRow+1)
+        for currentColumn in 0...7 {
+            if (myQueens[currentRow].column == currentColumn) {
+                print(" Q ")
+            } else {
+                print(" . ")
+            }
+        }
+        println()
+    }
+    println("  A  B  C  D  E  F  G  H  ")
+    println()
+}
+
+// start by trying to move queen across row 0
+moveQueenAcrossRow(0)
+
+// that's it
+println("POSITIONS CHECKED: \(positionsChecked)")