stack应用之逆波兰

stack

import sys

class Stack:
        def __init__(self):
                self.items=[]

        def push(self,item):
                try:
                        self.items.append(item)
                except:
                        print sys.exc_info()


        def pop(self):
                try:
                        self.items.pop()
                except:
                        print sys.exc_info()

        def clear(self):
                try:
                        del self.items[:]
                except:
                        print sys.exc_info()

        def empty(self):
                return len(self.items)==0

        def size(self):
                return len(self.items)

        def top(self):
                try:
                        return self.items[self.size()-1]
                except:
                        print sys.exc_info()

逆波兰

import stack

def evalRPN(tokens):
        operators="+-*/"

        stacktemp=stack.Stack()

        for item in tokens:
                #print item,"======================"
                #print stacktemp.items
                if operators.find(item)==-1:
                        stacktemp.push(item)
                else:
                        leftn=int(stacktemp.top())
                        stacktemp.pop()
                        rightn=int(stacktemp.top())
                        stacktemp.pop()
                        if item=='+':
                                stacktemp.push(str(leftn+rightn))
                        elif item=='-':
                                stacktemp.push(str(rightn-leftn))
                        elif item=='*':
                                stacktemp.push(str(leftn*rightn))
                        elif item=='/':
                                stacktemp.push(str(rightn/leftn))

        returnValue=int(stacktemp.top())
        stacktemp.pop()
        return returnValue


if __name__=="__main__":
        tokens= ["2", "1", "+", "3", "*"]
        print evalRPN(tokens)