Challenge 224 hard - Langford strings

ch224h.go

package main

import (
	"log"
)

var (
	n    int          = 8
	l    []int        = make([]int, n*2)
	used map[int]bool = make(map[int]bool)
)

func fill(pos int) {
	if pos >= n*2 {
		log.Println("Victory!", l)
		return
	} else if l[pos] != 0 { // Has a number at this position.
		fill(pos + 1)
	} else {
		for i := 1; i <= n; i++ {
			is_used, ok := used[i]
			if ok && is_used { // This number was used already.
				continue
			}

			second_pos := pos + i + 1
			if second_pos >= n*2 || l[second_pos] != 0 { // The right part would not fit.
				continue
			}

			// Place number.
			l[pos] = i
			l[pos+i+1] = i
			used[i] = true

			fill(pos + 1)

			// Remove number
			l[pos] = 0
			l[pos+i+1] = 0
			used[i] = false
		}
	}
}

func main() {
	fill(0)
}