Simple Expression Calculator

Problem Statement

Create a simple calculator that can evaluate arithmetic expressions given as strings.

The expressions will only include non-negative integers and the operators +, -, *, /, along with optional spaces. Division should truncate results towards zero.

You can assume the input expression is always valid and no built-in evaluation functions should be used.

Examples

Example 1:
Input: "4+3*5"
Output: 19

Example 2:
Input: " 8 / 4 "
Output: 2

Example 3:
Input: " 10 + 6 / 3 "
Output: 12

Constraints

• Expression length ranges from 1 to 250 characters.
• Only non-negative integers less than 10000 are used.
• Operators include +, -, *, / only.
• Spaces can appear anywhere in the expression.