Notebook

Basic Calculator¶

Problem¶

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¶

In [1]:

class Solution:
    def calculate(self, s: str) -> int:
        stack = []
        num = 0
        sign = 1
        result = 0

        for char in s:
            if char.isdigit():
                num = num * 10 + int(char)
            elif char == "+":
                result += sign * num
                num = 0
                sign = 1
            elif char == "-":
                result += sign * num
                num = 0
                sign = -1
            elif char == "(":
                stack.append(result)
                stack.append(sign)
                result = 0
                sign = 1
            elif char == ")":
                result += sign * num
                num = 0
                result *= stack.pop()  # Pop the sign.
                result += stack.pop()  # Pop the preceding result.

        result += sign * num
        return result


# Example usage:
solution = Solution()
s = "(1+(4+5+2)-3)+(6+8)"
result = solution.calculate(s)
print(result)  # Output: 23

Credits¶

I would like to thank Dhirajpatra for writing this piece of code. Feel free to reach him out if you have any questions about Python Programming.