1 """
2 Implement a basic calculator to evaluate a simple expression string.
3 The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.
4 Example 1:
5 Input: "3+2*2"
6 Output: 7
7 Example 2:
8 Input: " 3/2 "
9 Output: 1
10 Example 3:
11 Input: " 3+5 / 2 "
12 Output: 5
13 """
14 """
15 这道题因为有空格字符，连续数字字符使得考虑情况有些复杂
16 为了保存好符号，用了一个sign变量，初始化为'+'，保证其正常入栈
17 """
18 class Solution:
19 def calculate(self, s: str) -> int:
20 if not s:
21 return '0'
22 stack = []
23 sign = '+' # !!!用一个sign实现将符号后置使用
24 num = 0
25 for i in range(len(s)):
26 if s[i].isdigit():
27 num = num * 10 + int(s[i]) # ord(s[i]) - ord('0')
28 if not s[i].isdigit() and s[i] != ' ' or i == len(s) - 1:
29 if sign == '-': # 注意这里是sign
30 stack.append(-num)
31 elif sign == '+':
32 stack.append(num)
33 elif sign == '*':
34 stack.append(stack.pop() * num)
35 else: # 因为python里向下取整,-1.5取整为-2
36 temp = stack.pop()
37 if temp // num < 0 and temp % num != 0:
38 stack.append(temp // num + 1)
39 else:
40 stack.append(temp // num)
41 sign = s[i]
42 num = 0
43 res = 0
44 for j in range(len(stack)):
45 res += stack[j]
46 return res
47 if __name__ == '__main__':
48 ans = Solution()
49 s = "14-3/2"
50 ans.calculate(s)
