Implement basic calculator with four operations

Q: Implement basic calculator with four operations

This question evaluates a candidate's ability to parse and evaluate arithmetic expressions, correctly apply operator precedence and left-to-right associativity, and handle integer arithmetic details such as truncating division, whitespace, and multi-digit numbers, and is commonly asked to assess implementation-level parsing and algorithmic reasoning. The category is Coding & Algorithms and the level of abstraction is practical application, focusing on coding an efficient evaluator that balances time and space constraints rather than purely conceptual theory.

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Coding & Algorithms questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master coding & algorithms interviews.

Question

Implement a basic arithmetic expression evaluator.

Problem

You are given a string s representing a mathematical expression containing non-negative integers and the operators '+', '-', '*', and '/'.

Compute and return the value of the expression.

Details & Assumptions

The expression:
- Contains only:
  - Non-negative integers in base 10 (may have multiple digits)
  - The operators '+' , '-' , '*' , '/'
  - Space characters ' ' (which should be ignored)
- Is always syntactically valid (no malformed input, no division by zero).
Operator precedence:
- Multiplication '*' and division '/' have higher precedence than addition '+' and subtraction '-' .
- Operators of the same precedence are evaluated from left to right (left-associative).
Division is integer division that truncates toward zero .
Parentheses do not appear in the input.

Function Signature (example)

You may implement a function in the language of your choice, for example:

int calculate(String s)

and return the evaluated integer result.

Examples

Example 1

Input: "3+2*2"
Output: 7
Explanation: 2*2 = 4 , then 3 + 4 = 7 .

Example 2

Input: " 14-3/2 "
Output: 13
Explanation: 3/2 truncates toward zero and becomes 1 , so 14 - 1 = 13 .

Example 3

Input: "100/10*3+5"
Output: 35
Explanation: 100/10 = 10 , 10*3 = 30 , then 30 + 5 = 35 .

Constraints

1 <= s.length <= 100000
The result always fits in a 32-bit signed integer.
Aim for:
- Time complexity: O(n) where n is the length of s .
- Extra space: O(1) or O(n) (excluding the input string).