N queens problem in Swift

NQueens.swift

// jacob schwartz | @isoiphone
// solves N queens problem in swift
// for N=8 there should be 92 possible solutions
// each solution is a list of numbers, these indicate the row to place queen for given column
// for example 0,4,7,5,2,6,1,3 maps to:
//  Q-------
//  ------Q-
//  ----Q---
//  -------Q
//  -Q------
//  ---Q----
//  -----Q--
//  --Q-----

var N = 8

var numSolutions = 0
var solution = [Int](count: N, repeatedValue: 0)
var inRow = [Bool](count: N, repeatedValue: false)
var inForwardDiag = [Bool](count: 2*N, repeatedValue: false)
var inBackDiag = [Bool](count: 2*N, repeatedValue: false)

func placeQueen(col: Int) {
    if col == N {
        let line = ",".join(solution.map{"\($0)"})
        println("\(++numSolutions): \(line)")
        return
    }
    
    for row in 0..<N {
        if !inRow[row] && !inForwardDiag[row+col] && !inBackDiag[row-col+N] {
            solution[col] = row
            
            inRow[row] = true
            inForwardDiag[row+col] = true
            inBackDiag[row-col+N] = true
            placeQueen(col+1)
            inRow[row] = false
            inForwardDiag[row+col] = false
            inBackDiag[row-col+N] = false
        }
        solution[col] = row
    }
}

placeQueen(0)