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[11/20] 224. Basic Calculator

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Given a string s representing a valid expression, implement a basic calculator to evaluate it, and return the result of the evaluation.

Note: You are not allowed to use any built-in function which evaluates strings as mathematical expressions, such as eval().

 

Example 1:

Input: s = "1 + 1"
Output: 2

Example 2:

Input: s = " 2-1 + 2 "
Output: 3

Example 3:

Input: s = "(1+(4+5+2)-3)+(6+8)"
Output: 23

 

Constraints:

  • 1 <= s.length <= 3 * 105
  • s consists of digits, '+''-''('')', and ' '.
  • s represents a valid expression.
  • '+' is not used as a unary operation (i.e., "+1" and "+(2 + 3)" is invalid).
  • '-' could be used as a unary operation (i.e., "-1" and "-(2 + 3)" is valid).
  • There will be no two consecutive operators in the input.
  • Every number and running calculation will fit in a signed 32-bit integer.
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Runtime: 229 ms, faster than 42.49% of Python3 online submissions for Basic Calculator.
Memory Usage: 18.1 MB, less than 9.85% of Python3 online submissions for Basic Calculator.
class Solution:
    def calculate(self, s: str) -> int:
        s = list(s)
        stack = []
        
        def calc(op, num):
            if op == "+":
                stack.append(num)
            elif op == "-":
                stack.append(-num)
        
        operators = ["+","-"]
        op = "+"
        num = 0
        
        for char in s+["+"]:
            
            if char in operators:
                calc(op, num)
                op = char
                num = 0
            elif char.isdigit():
                num = num*10 + int(char)
            
            elif char == "(":
                stack.append(op)
                op = "+"
                num = 0
                
            elif char == ")":
                calc(op, num)
                op = "+"
                num = 0
                temp = 0
                while stack[-1] not in operators:
                    temp += stack.pop()
                
                temp_op = stack.pop()
                calc(temp_op, temp)
                
        return sum(stack)
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