local N = 8 --board size

local function isplaceok(a, n, c)
    for i = 1, n - 1 do
                if (a[i] == c) or
                   (a[i] - i == c - n) or 
                   (a[i] + i == c + n) then
                return false 
            end
        end
    return true
end

local function printsolution(a)
        for i = 1, N do
            for j = 1, N do
                io.write(a[i] == j and "X" or "-", "  ")
            end
            io.write("\n")
        end
        io.write("\n")
    end

local function addqueen(a, n)
        if n > N then
            printsolution(a)
        else
            for c = 1, N do
                if isplaceok(a, n, c) then
                        a[n] = c
                        addqueen(a, n + 1)
                    end
                end
            end
        end

addqueen({}, 1)