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 =selfsetmetatable(o,self)data[o] = {}return o
end
function DualStack:push(v)table.insert(data[self],v)
endfunction DualStack:pop()local v =  data[self][#data[self]]table.remove(data[self],#data[self])return v
endfunction DualStack:top()local v =  data[self][#data[self]]return v
end
function DualStack:isempty()return #data[self] == 0
endlocal stack = DualStack:new()
for i = 1, 10 dostack:push(i)
end
print(stack:top())
while not stack:isempty() doprint(stack:pop())
end