Practice: N-Queen

-- 在虚拟机中运行时提前定义N
if (type(N) ~= "number") then
    print("Have not defined a number 'N' previously. Now N = 8")
    N = 8
end

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

local function printBoard(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
        printBoard(a)
    else
        for i = 1, N do
            if (isPlaceOk(a, n, i)) then
                a[n] = i
                AddQueen(a, n + 1)
            end
        end
    end
end

AddQueen({}, 1)