chapter2.md

2 小插曲：八皇后问题

N = 8 -- 棋盘大小

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

-- 打印棋盘
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

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)