LeetCode 227. Basic Calculator II

题意：

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7
" 3/2 " = 1
" 3+5 / 2 " = 5

 

Note: Do not use the eval built-in library function.

思路：

这道题刚开始分+ - * /四种情况处理，最后发现情况太多，处理不了。

后来想下， 把乘除的当成一块。 那么问题会分成两个问题， 一个是处理加减，一个是处理乘除。

AC代码：

 1 class Solution(object):
 2     def mul_div(self, s):
 3         n = len(s)
 4         for i in xrange(1, n):
 5             if s[i] == "*":
 6                 for j in xrange(i+1, n+1):
 7                     if j==n:
 8                         return int(s[:i]) * int(s[i+1:j]) 
 9                     if s[j] == "+" or s[j] == "-" or s[j] == "*" or s[j] == "/":
10                         ss = str( int(s[:i]) * int(s[i+1:j]) )
11                         return self.mul_div( ss  + s[j:])
12             
13             if s[i] == "/":
14                 for j in xrange(i+1, n+1):
15                     if j==n:
16                         return int(s[:i]) / int(s[i+1:j]) 
17                     if s[j] == "+" or s[j] == "-" or s[j] == "*" or s[j] == "/":
18                         ss = str( int(s[:i]) / int(s[i+1:j]) )
19                         return self.mul_div( ss  + s[j:])
20                         
21         return int(s)
22         
23     def calculate(self, s):
24         """
25         :type s: str
26         :rtype: int
27         """
28         s = s.replace(" ", "")
29         n = len(s)
30         ans = 0
31         flag = 1
32         ss = ""
33         for i in range(n):
34             if s[i] == "+":
35                 ans += flag * self.mul_div(ss)
36                 ss = ""
37                 flag = 1
38             elif s[i] == "-":
39                 ans += flag * self.mul_div(ss)
40                 ss = ""
41                 flag = -1
42             else :
43                 ss += s[i]
44         ans += flag * self.mul_div(ss)
45         
46         return ans

 

posted @ 2016-08-09 00:14  Gu Feiyang  阅读(123)  评论(0)    收藏  举报