LeetCode - Basic Calculator

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.

 

ChatGPT's Solution:

class Solution(object):
    def calculate(self, s):
        """
        :type s: str
        :rtype: int
        """
        
        def evaluate(tokens):
            stack = []
            num = 0
            sign = 1  # 1 represents '+', -1 represents '-'

            for token in tokens:
                if token.isdigit():
                    num = num * 10 + int(token)
                elif token in '+-':
                    stack.append(sign * num)  # push the left operand into stack
                    num = 0
                    sign = 1 if token == '+' else -1
                elif token == '(':
                    stack.append(sign)
                    stack.append('(')
                    sign = 1
                    num = 0
                elif token == ')':
                    stack.append(sign * num)
                    num = 0
                    temp = 0

                    while stack and stack[-1] != '(':
                        temp += stack.pop()
                    stack.pop()  # remove '('
                    sign = stack.pop() if stack and isinstance(stack[-1], int) else 1
                    stack.append(sign * temp)
            
            stack.append(sign * num)
            return sum(stack)
        
        tokens = [char for char in s if char != ' ']

        return evaluate(tokens)