Skip to content

224. Basic Calculator

Difficulty Topics

Description

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.

Solution

basic-calculator.py
class Solution:
    def calculate(self, s: str) -> int:
        stack = []
        sign = 1
        result = number = 0

        for c in s:
            if c.isdigit():
                number = number * 10 + int(c)
            elif c == '+':
                result += sign * number
                number = 0
                sign = 1
            elif c == '-':
                result += sign * number
                number = 0
                sign = -1
            elif c == '(':
                stack.append(result)
                stack.append(sign)

                result = 0
                sign = 1
            elif c == ')':
                result += sign * number
                number = 0
                result *= stack.pop()
                result += stack.pop()

        if number != 0: 
            result += sign * number

        return result