[leetcode 150] Evaluate Reverse Polish Notation

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于 2015-01-12 11:31:43 发布

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分类专栏： [LeetCode 刷题] 文章标签： leetcode Stack

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本文介绍如何解析并计算逆波兰表达式的值，涉及基本运算符如加、减、乘、除，并通过实例演示计算过程。

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

class Solution:
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        stack = []
        lens = len(tokens)
        if lens == 0:
            return stack
        for i in xrange(lens):
            if tokens[i] == '+' or tokens[i] == '-' or tokens[i] == '*' or tokens[i] == '/':
                b = int(stack[-1])
                del stack[-1]
                a = int(stack[-1])
                del stack[-1]
                if tokens[i] == '+':
                    stack.append(str(a+b))
                elif tokens[i] == '-':
                    stack.append(str(a-b))
                elif tokens[i] == '*':
                    stack.append(str(a*b))
                elif tokens[i] == '/':
                    stack.append(str(a/b if a*b>0 else -((-a)/b)))
            else:
                stack.append(tokens[i])
        return int(stack[0])