Leetcode Basic Calculator Ii

Analyze and evaluate string-based mathematical expressions with stack logic

What is LeetCode Basic Calculator II?

The leetcode basic calculator ii is a classic algorithmic challenge found on the LeetCode platform (Problem #227). It tasks developers with implementing a basic calculator to evaluate a string expression containing non-negative integers and basic operators: addition (+), subtraction (-), multiplication (*), and division (/).

This problem is a cornerstone for learning about string expression evaluation and how computers interpret mathematical syntax. Unlike simpler calculators, the leetcode basic calculator ii requires strict adherence to the mathematical order of operations (MDAS – Multiplication, Division, Addition, Subtraction), even though it does not involve parentheses.

Who should use this tool? Competitive programmers, computer science students, and software engineers preparing for technical interviews find the leetcode basic calculator ii logic essential for mastering stack-based parsing and linear time complexity algorithms.

leetcode basic calculator ii Formula and Mathematical Explanation

The logic behind solving leetcode basic calculator ii revolves around a single-pass algorithm using a stack. Because multiplication and division have higher precedence than addition and subtraction, we must delay the evaluation of the latter until we confirm no higher-precedence operations follow.

Step-by-Step Algorithm Derivation:

1. Initialize a stack to store processed numbers.
2. Iterate through the string, parsing multi-digit numbers.
3. Keep track of the “last seen” operator (defaults to ‘+’).
4. When an operator or the end of the string is reached:
   • If the last operator was ‘+’, push the current number to the stack.
   • If ‘-‘, push the negative of the current number.
   • If ‘*’, pop the top of the stack, multiply it by the current number, and push the result back.
   • If ‘/’, pop the top of the stack, divide it by the current number (integer division), and push the result back.
5. The final result is the sum of all elements in the stack.

Variable Table

Variable	Meaning	Data Type	Role
s	Input string	String	Contains digits and operators
stack	Processing container	Array/Stack	Stores intermediate values for addition
currentNum	Current parsed integer	Integer	Accumulates digits like ‘1’ and ‘2’ into 12
lastOp	Previous operator	Character	Determines how currentNum enters the stack

Practical Examples (Real-World Use Cases)

Example 1: High Precedence at the End

Expression: 3 + 2 * 2

• Process ‘3’: lastOp is ‘+’, push 3. Stack: [3].
• Process ‘2’: lastOp is ‘+’, push 2. Stack: [3, 2].
• Process ‘*’: Current operator is ‘*’, next number is 2. Pop 2, multiply by 2, push 4. Stack: [3, 4].
• Final Result: 3 + 4 = 7.

Example 2: Handling Integer Division

Expression: 10 - 5 / 2

• Process ’10’: Push 10. Stack: [10].
• Process ‘-‘: Next number is 5. lastOp becomes ‘-‘.
• Process ‘/’: Next number is 2. Since lastOp was ‘/’, pop 5, divide by 2 (integer division = 2), push -2. Stack: [10, -2].
• Final Result: 10 + (-2) = 8.

How to Use This leetcode basic calculator ii Calculator

1. Enter Expression: Type your mathematical string into the input box. You can use spaces as the calculator automatically ignores them.
2. Real-time Validation: The calculator updates as you type. If you enter an invalid character, an error message will appear.
3. Review the Stack: Look at the “Stack State” in the intermediate values section to see how the numbers were processed.
4. Analyze the Steps: Consult the trace table to see exactly what happened at every character index. This is vital for understanding the leetcode basic calculator ii logic.
5. Export Results: Click “Copy Results” to save the calculation for your documentation or code comments.

Key Factors That Affect leetcode basic calculator ii Results

• Operator Precedence: Multiplication and division must be handled immediately by modifying the stack’s top element, while addition/subtraction are deferred.
• Whitespace Handling: The leetcode basic calculator ii requires ignoring spaces, which can exist anywhere in the string.
• Integer Division: In many languages (and the LeetCode prompt), division must truncate toward zero. Our tool follows this standard.
• Multi-digit Integers: The algorithm must correctly combine consecutive characters (e.g., ‘1’, ‘0’, ‘0’) into a single integer (100).
• Stack Depth: The space complexity is O(n). While rare in simple expressions, very long strings of additions can lead to significant stack memory usage.
• Execution Order: The algorithm reads from left to right. Even though multiplication happens first logically, the parser reads every character in sequence once.

Frequently Asked Questions (FAQ)

Does this calculator support parentheses?

No, the leetcode basic calculator ii specifically focuses on MDAS without parentheses. For parentheses, “Basic Calculator I” logic is required.

What is the time complexity?

The leetcode basic calculator ii algorithm runs in O(n) time, where n is the length of the string, as it visits each character once.

How are negative numbers handled?

The input usually contains non-negative integers. However, the subtraction operator creates negative values inside the stack to facilitate final summation.

What happens with division by zero?

In a standard leetcode basic calculator ii implementation, division by zero is undefined and would typically throw an error.

Why use a stack instead of eval()?

In coding interviews, using eval() is generally prohibited as it bypasses the need to demonstrate knowledge of stack-based parsing and order of operations algorithm design.

Does it handle decimal numbers?

Standard leetcode basic calculator ii constraints only involve integers, so this tool performs integer division.

Can the stack overflow?

With a standard string expression, the stack size is proportional to the number of terms, making overflow extremely unlikely for typical string lengths.

What is the difference between this and Calculator III?

Calculator II adds multiplication/division. Calculator III typically adds parentheses and negative number support to the mix.

Related Tools and Internal Resources

• Comprehensive String Parsing Guide: Learn the basics of character-by-character iteration.
• Algorithm Efficiency Calculator: Analyze O(n) vs O(log n) performance.
• Data Structures: Stack Tutorial: A deep dive into LIFO principles.
• Postfix (Reverse Polish) Notation Converter: Convert infix expressions for easier calculation.
• Coding Interview Preparation: Roadmap for acing algorithmic interviews.
• Complexity Analysis Tools: Visualize space and time growth.