T pop()
{//出队
if (B.empty())
{//若B空，则队首元素应该在A的栈底
while (!A.empty())//将A中元素全部压入B
B.push(A.pop());
}
return B.pop();//出B栈
}
void pop()
{
if (head.empty())
{
while (!tail.empty())
{
head.push(tail.top());
tail.pop();
}
}
head.pop();
}
