栈遵循LIFO ( last in first out) 即后入先出原则
栈结构类似于叠盘子 后叠上去的要先拿走 才能拿到下面的盘子
因此stack是一种访问受限的线性存储结构
用单向链表的结构来存储
stack类
class stack
{
stack();
bool empty() const;
void pop(); //移除栈顶元素
void push()(const T&item);//增加元素至栈顶
int size() const;
T &top() const;//返回栈顶元素
}
栈的两个常见应用：
&&Run-time Stack 运行时间栈
顺序执行活动，运行栈顶的活动，运行完毕弹出。
&&RPN or Postfix Notation
将中缀表达式转化为后缀表达式压入栈中，顺序弹出进行运算。
如：a*b+c -> ab*c+
a*(b+c) -> abc+*
a-(b-(c-d)) -> abcd---
压入栈中时分为 数字栈 和 符号栈
符号栈：进栈与栈顶元素比较优先级。
同级则先弹出栈顶元素再新元素进栈，
优先级低则先弹出栈顶元素再与新的栈顶元素比较，
优先级高则直接入栈。
‘（’ 直接入栈，碰到 ‘）’ 则将直至 ‘（’ 的符号全部弹出。
举例：
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" 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