Basic Calculator

Code Pattern

Get Number

get_number: Reads and converts a sequence of digits from the string into an integer.

Update Stack

update_stack: Modifies the stack based on the arithmetic operation specified, enabling the evaluation of expressions in a structured manner.
- Addition: If the operator is +, it appends num to the stack.
- Subtraction: If the operator is -, it appends -num to the stack to account for the negative value.
- Multiplication: If the operator is *, it pops the top value from the stack, multiplies it by num, and appends the result back to the stack.
- Division: If the operator is /, it pops the top value, performs integer division with num, and appends the result back to the stack (using int to ensure integer division).

Calculate

Character Processing
- If the character is a digit, it retrieves the full number.
- If it encounters a (, it recursively calls calc() to evaluate the expression inside.
- If it sees a ), it updates the stack with the current num and operator, then returns the sum of the stack.
- For operators (+, -, *, /), it updates the stack with the current number and operator, sets the new operator, and resets num.

def calculate(self, s: str) -> int:
    self.index = 0
    def get_number():
        num = 0
        while self.index < len(s) and s[self.index].isdigit():
            num = num * 10 + int(s[self.index])
            self.index += 1
        return num

Common mistakes

Forget to reset num after an operator

# Wrong: Forgetting to reset num after processing an operator
elif char in '+-*/':
    update_stack(stack, num, operator)
    # num is not reset here, leading to incorrect calculations
# Correct:
elif char in '+-*/':
    update_stack(stack, num, operator)
    operator = char
    num = 0  # Reset num for the next number

224. Basic Calculator

class Solution:
    def calculate(self, s: str) -> int:
        self.index = 0
        
        def get_number():
            # Use the code above
        
        def update_stack(stack, num, operator):
            # Use the code above
        
        def calc():
            # Use the code above
            
        return calc()

227. Basic Calculator II

class Solution:
    def calculate(self, s: str) -> int:
        self.index = 0
        
        def get_number():
            # Use the code above
        
        def update_stack(stack, num, operator):
            # Use the code above
        
        def calc():
            # Use the code above
            
        return calc()

227. Basic Calculator III

class Solution:
    def calculate(self, s: str) -> int:
        self.index = 0
        
        def get_number():
            # Use the code above
        
        def update_stack(stack, num, operator):
            # Use the code above
        
        def calc():
            # Use the code above
            
        return calc()

Basic Calculator

Code Pattern

Get Number

Update Stack

Calculate

Common mistakes

Related Problem