N = 8  -- board size

function isplaceok(a, n, c)
  for i = 1, n - 1 do -- for each queen already placed
    if (a[i] == c) or -- same column?
       (a[i] - i == c - n) or -- same diagonal?
       (a[i] + i == c + n) then -- same diagonal?
       return false -- place can be attacked
    end
  end
  return true -- no attacks; place is OK
end

-- print a board
function printsolution(a)
  for i = 1, N do --for each row
    for j = 1, N do -- and for each column
      -- writes "X" or "-" plus space
      io.write(a[i] == j and "X" or "-"," ")
    end
    io.write("\n")
  end
  io.write("\n")
end

-- add to board 'a' all queens from 'n' to 'N'
function addqueen(a, n)
  if n > N then -- all queens have been placed?
    printsolution(a)
  else -- try to place the n-th queen
    for c = 1, N do
      if isplaceok(a, n, c) then
        a[n] = c -- place the n-th queen at column 'c'
        addqueen(a, n+1)
      end
    end
  end
end

-- run the program
addqueen({}, 1)