leetcode : 227. Basic Calculator II

题目

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.

Example 1:

Input: "3+2*2"
Output: 7

Example 2:

Input: " 3/2 "
Output: 1

Example 3:

Input: " 3+5 / 2 "
Output: 5

Note:

• You may assume that the given expression is always valid.
• Do not use the eval built-in library function.

思路

1、定义一个计算乘除的函数，字符串只包含 * / 运算

2、将字符串先按照 + 分开

3、如果片段中有 -，按照 - 拆分。对每个拆分的片段，先运行乘除计算，然后将得到的值加起来

答案

class Solution:
    def calculate(self, s: str) -> int:
        def multiply(string):
            nums = string.split("*")
            ans = 1
            for n in nums:
                if "/" in n:
                    divides = n.split("/")
                    ans *= int(divides[0])
                    for i in range(1,len(divides)):
                        ans = ans//int(divides[i])
                else:
                    ans *= int(n)
            return ans
        
        s = s.replace(" ","")
        s = s.split("+")
        ans = 0
        for num in s:
            if "-" in num:
                num_reduce = num.split("-")
                temp_reduce = multiply(num_reduce[0])
                for i_r in range(1,len(num_reduce)):
                    temp_reduce -= multiply(num_reduce[i_r])
                ans += temp_reduce
            else:                
                ans += multiply(num)
        return ans                
        

ps:

答案里有个思路是用 栈，也不错

 

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