/*
 * File: nQueens.kt
 * Purpose: Solves the n-queens puzzle.
 * Compilation: kotlinc nQueens.kt
 * Execution: kotlin NQueensKt n
 *   where n is a positive integer
 * Author: San Skulrattanakulchai
 */

import kotlin.math.abs

fun main(args: Array<String>) {

    fun swap(a: IntArray, i: Int, j: Int) {
        val tmp = a[i]
        a[i] = a[j]
        a[j] = tmp
    }

    fun queen(size: Int) {
        val board = Array<CharArray>(size) { CharArray(size) { ' ' } }
        val pi = IntArray(size) { it }

        fun printBoard() {
            val line = "${"+---".repeat(size)}+\n"
            val sb = StringBuilder()
            for (i in 0 until size) {
                sb.append(line)
                for (j in 0 until size)
                    sb.append("| ${board[i][j]} ")
                sb.append("|\n")
            }
            sb.append(line)
            print(sb)
        }

        fun isCompat(i: Int, j: Int): Boolean {
            fun isAttacking(x1: Int, y1: Int, x2: Int, y2: Int) =
                x1 == x2 || y1 == y2 || abs(x1-x2) == abs(y1-y2)
            for (k in 0 until i) {
                if (isAttacking(k, pi[k], i, j))
                    return false
            }
            return true
        }

        fun perm(i: Int): Boolean {
            if (i == size) {
                println("Here is one possible answer.")
                for (j in 0 until size)
                    board[j][pi[j]] = 'Q'
                printBoard()
                return true
            } else {
                for (j in i until size) {
                    if (isCompat(i, pi[j])) {
                        swap(pi, i, j)
                        if (perm(i+1))
                            return true
                        swap(pi, i, j)
                    }
                }
                return false
            }
        }

        val randGen = java.util.Random()
        for (i in size downTo 1)
            swap(pi, i-1, randGen.nextInt(i))

        if (!perm(0))
            println("There is no answer.")
    }

    queen(args[0].toInt())
}