【LeetCode with Python】 Evaluate Reverse Polish Notation

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原题页面：https://oj.leetcode.com/problems/evaluate-reverse-polish-notation/
题目类型：栈
难度评价：★★
本文地址：http://blog.csdn.net/nerv3x3/article/details/3465981

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

逆波兰式的计算。相信大多数计算机和软件专业的朋友大学时都做过的算法题，用栈模拟计算过程。

class Solution:    # @param tokens, a list of string    # @return an integer    def evalRPN(self, tokens):        len_tokens = len(tokens)        if 0 == len_tokens:            return 0        # python has no stack, so we have to simulate it        nums_stack = []        for i in range(0, len_tokens):            token = tokens[i]            # consider negative number, so shouldn't decide by the first char, but last char            last_ch = token[len(token) - 1]            if last_ch >= '0' and last_ch <= '9':                nums_stack.append(int(token))            else:                right = nums_stack.pop()                left = nums_stack.pop()                if '+' == token:                    result = left + right                elif '-' == token:                    result = left - right                elif '*' == token:                    result = left * right                else:                    sign = 1 if (left >= 0 and right >= 0) or (left <= 0 and right <= 0) else -1                    result = abs(left) // abs(right) * sign                nums_stack.append(result)        return nums_stack.pop()