使用对偶表示重新实现Stack（lua程序设计21.7练习21.3题）

local data = {}
DualStack = {}
function DualStack:new(o)
    o = o or {}
    self.__index =self
    setmetatable(o,self)
    data[o] = {}
    return o
end
function DualStack:push(v)
    table.insert(data[self],v)
end

function DualStack:pop()
    local v =  data[self][#data[self]]
    table.remove(data[self],#data[self])
    return v
end

function DualStack:top()
    local v =  data[self][#data[self]]
    return v
end
function DualStack:isempty()
    return #data[self] == 0
end

local stack = DualStack:new()
for i = 1, 10 do
    stack:push(i)
end
print(stack:top())
while not stack:isempty() do
    print(stack:pop())
end