func N_queens_solution(N = 8) {

    func collision(field, row) {
        for i in (^row) {
            var distance = (field[i] - field[row])
            distance ~~ [0, row-i, i-row] && return true
        }
        return false
    }

    func search(field, row) {
        row == N && return field
        for i in (^N) {
            field[row] = i
            if (!collision(field, row)) {
                return (__FUNC__(field, row+1) || next)
            }
        }
        return []
    }

    for i in (0 .. N>>1) {
        if (var r = search([i], 1)) {
            return r
        }
    }
}

for n in (1..15) {
    say "#{'%2d' % n}: #{N_queens_solution(n) || 'No solution'}"
}