#Leet Code# Evaluate Reverse Polish Notation

描述：计算逆波兰表达法的结果

Sample：

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

使用stack实现：

 1 def is_op(c):
 2     return c in [‘+‘, ‘-‘, ‘*‘, ‘/‘]
 3
 4 def divide(x, y):
 5     if (x * y) < 0:
 6         return -1 * ((-x)/y)
 7     return x/y
 8
 9 class Solution:
10     # @param tokens, a list of string
11     # @return an integer
12     def evalRPN(self, tokens):
13         opDict = {‘+‘: lambda x,y: x+y,
14                   ‘-‘: lambda x,y: x-y,
15                   ‘*‘: lambda x,y: x*y,
16                   ‘/‘: divide}
17         record = []
18
19         for item in tokens:
20             if is_op(item):
21                 second = record.pop()
22                 first = record.pop()
23                 record.append(opDict[item](first, second))
24             else:
25                 record.append(int(item))
26         return record[0]

使用树实现：

Todo

备注：python的除法跟c++不太一样 3/-5 = -1

结论：

根据问题的具体特性，选择合适的数据结构解决问题会差别很大

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