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This is a Medium difficulty Coding & Algorithms question, commonly asked during Onsite rounds at Meta.

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This question is commonly asked for Software Engineer candidates at Meta during technical interviews.

Implement basic expression evaluator

Company: Meta

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: Medium

Interview Round: Onsite

Implement a function to evaluate a string expression containing non-negative integers, '+', '-', '*', '/', and spaces. Multiplication and division have higher precedence than addition and subtraction, and division truncates toward zero. Parentheses are not allowed. Do not use any built-in eval. Handle long inputs efficiently and document time and space complexity.

Quick Answer: This question evaluates proficiency in expression parsing, operator precedence handling, integer arithmetic semantics (including truncating division), and efficient tokenization and evaluation within the Coding & Algorithms domain.

Implement a function `evaluate` that evaluates a string expression containing non-negative integers, the operators `+`, `-`, `*`, `/`, and spaces. Multiplication and division have higher precedence than addition and subtraction. Integer division truncates toward zero. Parentheses are NOT allowed, and you may not use any built-in `eval`. The input is guaranteed to be a valid expression that fits in a 32-bit signed integer at every intermediate step. Example 1: Input: s = "3+2*2" Output: 7 Example 2: Input: s = " 3/2 " Output: 1 Example 3: Input: s = " 3+5 / 2 " Output: 5

Constraints

1 <= s.length <= 3 * 10^5
s consists of non-negative integers and the characters '+', '-', '*', '/', and ' '.
s represents a valid expression.
All intermediate values fit in a signed 32-bit integer.
The integer division truncates toward zero.
Parentheses are not allowed; do not use any built-in eval.

Examples

Input: "3+2*2"

Expected Output: 7

Explanation: 2*2=4 binds tighter than +, so 3+4=7.

Input: " 3/2 "

Expected Output: 1

Explanation: Spaces are ignored; 3/2 truncates to 1.

Input: " 3+5 / 2 "

Expected Output: 5

Explanation: 5/2=2 first, then 3+2=5.

Input: "14-3/2"

Expected Output: 13

Explanation: 3/2=1, then 14-1=13.

Input: "100"

Expected Output: 100

Explanation: A single number evaluates to itself.

Input: "2*3*4"

Expected Output: 24

Explanation: Left-to-right multiplication: (2*3)*4=24.

Input: "10-2*3+4/2"

Expected Output: 6

Explanation: 2*3=6 and 4/2=2, then 10-6+2=6.

Input: "0"

Expected Output: 0

Explanation: Boundary: zero.

Input: "1+1+1+1"

Expected Output: 4

Explanation: Pure addition stresses the stack worst case.

Input: "7/3"

Expected Output: 2

Explanation: 7/3=2.33 truncates toward zero to 2.

Hints

Track a running operator (start as '+') and accumulate the current number digit by digit. Apply the operator when you hit the next operator or the end of the string.
Use a stack: for '+' push +num, for '-' push -num. For '*' and '/', pop the top value and combine immediately — that resolves the higher precedence on the fly.
The final answer is just the sum of everything left on the stack — no second pass needed.
For truncate-toward-zero division with negatives on the stack, divide magnitudes and re-apply the sign (Python's // floors, so handle the sign explicitly).

Quick Overview

This question evaluates proficiency in expression parsing, operator precedence handling, integer arithmetic semantics (including truncating division), and efficient tokenization and evaluation within the Coding & Algorithms domain.