Leetcode Basic Calculator Ii

This calculator provides a solution to the leetcode basic calculator ii problem (LeetCode 227). Enter a string expression containing non-negative integers and the operators +, -, *, / to see the evaluated result, respecting standard operator precedence.

What is the leetcode basic calculator ii problem?

The leetcode basic calculator ii (LeetCode problem 227) is a popular coding challenge that asks you to implement a calculator to evaluate a simple mathematical expression string. The expression contains non-negative integers and the four basic arithmetic operators: addition (+), subtraction (-), multiplication (*), and division (/). A key requirement is to correctly handle operator precedence, where multiplication and division are evaluated before addition and subtraction. The integer division must truncate toward zero.

This problem is an excellent exercise for developers looking to practice string parsing, handling edge cases, and implementing algorithms that manage state, such as a stack based calculator. It’s frequently asked in technical interviews for software engineering roles because it tests fundamental computer science concepts without requiring obscure knowledge.

Who Should Use This Tool?

• Software Developers: Anyone preparing for coding interviews who wants to verify their understanding of the leetcode basic calculator ii solution.
• Students: Computer science students learning about data structures (like stacks) and algorithms for parsing and evaluation.
• Hobbyists: Anyone curious about how programming languages interpret and compute mathematical formulas.

Common Misconceptions

A common mistake when first approaching the leetcode basic calculator ii problem is to simply evaluate the expression from left to right. This fails because it ignores operator precedence. For example, 3 + 2 * 2 would incorrectly result in 10 instead of the correct answer, 7. The core challenge is implementing an algorithm that respects the standard order of operations.

LeetCode Basic Calculator II Formula and Algorithm Explanation

There is no single “formula” for the leetcode basic calculator ii, but rather a well-defined algorithm. The most common and intuitive solution uses a stack to manage numbers and operations. The algorithm processes the expression string character by character, applying operations based on their precedence.

The core logic is as follows: scan the string, building up a number as you go. When you encounter an operator or reach the end of the string, you process the number you’ve just parsed based on the *previous* operator seen.

1. If the previous operator was +, push the current number onto the stack.
2. If the previous operator was -, push the negative of the current number onto the stack.
3. If the previous operator was *, pop the top value from the stack, multiply it by the current number, and push the result back onto the stack.
4. If the previous operator was /, pop the top value from the stack, perform integer division with the current number, and push the result back onto the stack.

After iterating through the entire string, the final result is the sum of all numbers remaining in the stack. This elegant approach ensures that higher-precedence operations (*, /) are resolved before the final summation of lower-precedence operations (+, -). This is a classic example of an operator precedence algorithm.

Algorithm Variables

Variable	Meaning	Unit	Typical Range
currentNumber	The number currently being parsed from the string.	Integer	Non-negative integers.
lastOperator	The operator encountered just before the currentNumber.	Character	+, -, *, /
stack	A data structure used to store intermediate values.	Array of Integers	Contains operands pending final summation.
result	The final evaluated value of the expression.	Integer	32-bit signed integer.

Practical Examples of LeetCode Basic Calculator II

Understanding the algorithm is easier with a few examples. Here’s how the leetcode basic calculator ii logic works in practice.

Example 1: Expression “3 + 5 / 2”

• Inputs: Expression String = "3 + 5 / 2"
• Step 1: Parse 3. Previous operator is + (default). Push 3 onto the stack. Stack: .
• Step 2: See +. Update last operator to +.
• Step 3: Parse 5. Previous operator is +. Push 5 onto the stack. Stack: .
• Step 4: See /. Update last operator to /.
• Step 5: Parse 2. Previous operator is /. Pop 5, calculate 5 / 2 = 2 (integer division), push 2. Stack: .
• Step 6: End of string. Sum the stack: 3 + 2 = 5.
• Output: 5

Example 2: Expression “4 * 5 – 10 / 2”

• Inputs: Expression String = "4 * 5 - 10 / 2"
• Step 1: Parse 4. Previous operator is + (default). Push 4. Stack: .
• Step 2: See *. Update operator to *.
• Step 3: Parse 5. Previous operator is *. Pop 4, calculate 4 * 5 = 20, push 20. Stack: .
• Step 4: See -. Update operator to -.
• Step 5: Parse 10. Previous operator is -. Push -10. Stack: [20, -10].
• Step 6: See /. Update operator to /.
• Step 7: Parse 2. Previous operator is /. Pop -10, calculate -10 / 2 = -5, push -5. Stack: [20, -5].
• Step 8: End of string. Sum stack: 20 + (-5) = 15.
• Output: 15

How to Use This LeetCode Basic Calculator II Calculator

Using this leetcode basic calculator ii tool is straightforward and provides instant results and insights.

1. Enter Expression: Type your mathematical expression into the “Expression String” input field. The expression should only contain numbers, +, -, *, /, and spaces.
2. View Real-Time Results: The calculator updates automatically as you type. The final answer appears in the large “Final Result” box.
3. Analyze Intermediate Values: Below the main result, you can see key intermediate values from the calculation, including the final state of the stack. This is crucial for debugging your own leetcode 227 solution.
4. Follow the Steps: The “Parsing and Calculation Steps” table breaks down how the algorithm processes your string token by token, showing how the stack and other variables change over time.
5. Reset or Copy: Use the “Reset” button to clear the input and start over, or “Copy Results” to save a summary of the calculation to your clipboard.

Key Factors That Affect LeetCode Basic Calculator II Results

Several factors influence the outcome of a leetcode basic calculator ii evaluation. Understanding them is key to implementing a correct solution.

• Operator Precedence: This is the most critical factor. The rules that define * and / as higher precedence than + and - dictate the order of operations and are the core of the problem. Incorrectly handling this leads to wrong answers, as seen in the 3 + 2 * 2 example.
• Integer Division: The problem specifies that division should truncate toward zero. This means 5 / 2 is 2, and -5 / 2 is -2. Floating-point division would produce incorrect results.
• Order of Same-Precedence Operators: For operators of the same precedence (e.g., 10 - 2 + 3), evaluation proceeds from left to right. The stack-based algorithm naturally handles this.
• White Space: The expression string can contain spaces, which must be ignored. The parsing logic should be able to handle strings like " 3 + 5 / 2 " correctly.
• Multi-Digit Numbers: The logic must be able to parse numbers with more than one digit (e.g., 123) before processing an operator.
• Expression Validity: The problem statement guarantees a valid expression, so you don’t need to handle malformed inputs like "3 + * 2". This simplifies the logic required.

Frequently Asked Questions (FAQ)

What is the time and space complexity of the stack solution?

The time complexity is O(n), where n is the length of the string, because we iterate through the string once. The space complexity is O(n) in the worst case (e.g., an expression like “1+2+3+4”), as the stack could potentially store all the numbers.

Why can’t I use the `eval()` function?

The purpose of the leetcode basic calculator ii problem is to test your ability to create a parsing and evaluation algorithm from scratch. Using a built-in function like `eval()` would bypass the entire challenge.

How does the algorithm handle subtraction?

A common trick is to treat subtraction as the addition of a negative number. When the `lastOperator` is -, you push the negative of the `currentNumber` onto the stack. This simplifies the final step to just summing all elements in the stack.

What if the expression starts with a negative number?

The problem statement specifies non-negative integers, so unary operators for the first number are not part of the standard test cases. The logic generally assumes a default + operator for the first number.

Can this calculator handle parentheses?

No. This calculator is specifically for the leetcode basic calculator ii problem, which does not include parentheses. Parentheses are introduced in Basic Calculator I and III, which require a more complex recursive or multi-stack approach.

How is this different from a shunting-yard algorithm?

The Shunting-yard algorithm is a more general method for converting an infix expression (like 3 + 4) to a postfix one (like 3 4 +), which is then easily evaluated. The stack method described here is a direct evaluation approach that combines parsing and calculation in a single pass, optimized for this specific problem’s constraints.

Does the order of multiplication and division matter?

Yes, they are evaluated left-to-right. For example, 10 / 2 * 5 is calculated as (10 / 2) * 5 = 25. The stack-based approach handles this correctly by processing operators as they appear.

Why is this problem so common in interviews?

It tests multiple core skills efficiently: string manipulation, understanding data structures (stacks), algorithmic thinking (operator precedence), and attention to detail with edge cases (integer division, multi-digit numbers). It’s a great proxy for general problem-solving ability.

Related Tools and Internal Resources

If you found this tool helpful, you might be interested in these other resources:

• Scientific Calculator: A full-featured calculator for more complex mathematical functions.
• Infix to Postfix Converter: A tool that demonstrates the shunting-yard algorithm for expression conversion.
• Understanding the Shunting-Yard Algorithm: A deep dive into the classic algorithm for parsing mathematical expressions.
• Stacks in Computer Science: An article explaining the fundamentals of the stack data structure and its common use cases.
• LeetCode Basic Calculator I Solution: An explanation of the solution for the version of the problem that includes parentheses.
• Guide to Operator Precedence: A guide on how different programming languages handle the order of operations.