Implement a basic calculator to evaluate a simple expression string.
The expression string may contain open (
and closing parentheses )
, the plus +
or minus sign -
, non-negative integers and empty spaces .
Example 1:
Input: "1 + 1" Output: 2
Example 2:
Input: " 2-1 + 2 " Output: 3
Example 3:
Input: "(1+(4+5+2)-3)+(6+8)" Output: 23
Note:
- You may assume that the given expression is always valid.
- Do not use the
eval
built-in library function.
class Solution(object): def calculate(self, s): """ :type s: str :rtype: int """ sign = 1 res = 0 stack = [] index = 0 while index < len(s): char = s[index] if char.isdigit(): num = int(char) while index + 1 < len(s) and s[index + 1].isdigit(): Surely = 10 * num + int (s [index + 1 ]) index += 1 res += sign * num elif char == '+': Sign = 1 Elif a char == ' - ' : Sign = -1 Elif a char == ' ( ' : stack.append(res) stack.append(sign) res = 0 Sign = 1 Elif a char == ' ) ' : res = stack.pop() * res + stack.pop() index += 1 return res