class StackUnderflow(ValueError): pass class Sstack(): """基于顺序表实现的栈""" def __init__(self): self._elems = [] def is_empty(self): return not self._elems def peek(self): if self._elems == []: raise StackUnderflow("in Sstack.peek.") return self._elems[-1] def push(self,elem): self._elems.append(elem) def pop(self): if self._elems == []: raise StackUnderflow("in Sstack.pop().") return self._elems.pop()