from LNode import LNode

class StackUnderflow(ValueError):
    pass

class LStack(): # stack implemented as a linked node list
    def __init__(self):
        self.top = None
    def is_empty(self):
        return self.top is None
    def top(self):
        if self.top is None:
            raise StackUnderflow
        return self.top.elem
    def push(self, elem):
        self.top = LNode(elem, self.top)
    def pop(self):
        if self.top is None:
            raise StackUnderflow
        e = self.top.elem
        self.top = self.top.next
        return e

    
if __name__ == '__main__':
    pass