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Evaluate a Four-Operator Expression | DoorDash Coding Question

Evaluate a Four-Operator Expression

Company: DoorDash

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: easy

Interview Round: Onsite

Implement a function that evaluates an arithmetic expression string containing non-negative integers, optional spaces, and the operators `+`, `-`, `*`, and `/`. There are no parentheses. Multiplication and division must be evaluated before addition and subtraction. Integer division should truncate toward zero. Function signature: `int evaluate(string s)` Examples: - `"3+2*2"` -> `7` - `" 14 - 3 / 2 "` -> `13` - `"18/3*2+5"` -> `17` Assume the input expression is always valid.

Quick Answer: This question evaluates expression-evaluation skills such as operator precedence, integer arithmetic semantics (truncating division), and string parsing/tokenization.

Implement a function that evaluates a valid arithmetic expression string containing non-negative integers, optional spaces, and the operators `+`, `-`, `*`, and `/`. There are no parentheses. Multiplication and division must be evaluated before addition and subtraction. Operators with the same precedence are evaluated from left to right. Integer division must truncate toward zero, so `-3 / 2` evaluates to `-1`.

Constraints

1 <= len(s) <= 200000
`s` contains only digits, spaces, and the operators `+`, `-`, `*`, `/`.
The expression is always valid and contains no parentheses.
All integer literals are non-negative.
Division by zero will not occur.
The final answer fits in a signed 32-bit integer.

Examples

Input: ("3+2*2",)

Expected Output: 7

Explanation: Multiplication is evaluated first: 2 * 2 = 4, then 3 + 4 = 7.

Input: (" 14 - 3 / 2 ",)

Expected Output: 13

Explanation: 3 / 2 truncates to 1, so the expression becomes 14 - 1 = 13.

Input: ("18/3*2+5",)

Expected Output: 17

Explanation: Division and multiplication have the same precedence and are evaluated left to right: 18 / 3 = 6, 6 * 2 = 12, then 12 + 5 = 17.

Input: ("42",)

Expected Output: 42

Explanation: A single number evaluates to itself.

Input: ("0-3/2",)

Expected Output: -1

Explanation: The subtraction creates a negative term: -3 / 2 truncates toward zero to -1.

Input: ("1+2*3-4/5+6",)

Expected Output: 13

Explanation: 2 * 3 = 6 and 4 / 5 truncates to 0, so the expression becomes 1 + 6 - 0 + 6 = 13.

Input: (" 100000 / 10 / 3 ",)

Expected Output: 3333

Explanation: Division is evaluated left to right: 100000 / 10 = 10000, then 10000 / 3 truncates to 3333.

Hints

When you finish reading a number, apply the previous operator to it rather than the current operator.
To handle precedence without parentheses, keep track of the most recent multiplication/division term separately from the committed addition/subtraction total.

Quick Overview

This question evaluates expression-evaluation skills such as operator precedence, integer arithmetic semantics (truncating division), and string parsing/tokenization.