Evaluate Reverse Polish Notation

Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9

["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

原题链接: https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/

题目大意: 逆波兰表达式

思路: 利用栈实现, 注意+, -单目运算符; 另外目前leetcode使用的python为2.7.5, 除法不是精确的除法, 两个整数相除是地板除, 浮点数除是精确的除法; 注意负数的判断, 使用isdigit是判断不了的;

 1 class Solution:

 2     # @param tokens, a list of string

 3     # @return an integer

 4     def evalRPN(self, tokens):

 5         stack = list()

 6 

 7         for token in tokens:

 8             try:

 9                 tmp = int(token)

10             except ValueError: #operator

11                 if token == '+': #may be a unary operator

12                     if len(stack) > 1:

13                         o1 = stack.pop()

14                         o2 = stack.pop()

15 

16                         stack.append(o2 + o1)

17                 elif token == '-':

18                     if len(stack) > 1:

19                         o1 = stack.pop()

20                         o2 = stack.pop()

21 

22                         stack.append(o2 - o1)

23                     else:

24                         o1 = stack.pop()

25                         stack.append(-o1)

26 

27                 elif token == '*':

28                     o1 = stack.pop()

29                     o2 = stack.pop()

30 

31                     stack.append(o2 * o1)

32                 else:

33                     o1 = stack.pop()

34                     o2 = stack.pop()

35 

36                     stack.append(int(float(o2) / o1))

37 

38             else: #number

39                 stack.append(tmp)

40 

41             

42         return stack[0]