Problem Description:
Given a string s representing a basic mathematical expression with non-negative integers and operators (+, -, *, /), evaluate the expression following the standard operator precedence:
Multiplication (*) and Division (/) have higher precedence than Addition (+) and Subtraction (-).
Division is integer division (truncate towards zero).
No parentheses are included.
Example:
Input: "3+7*7-9/3"
Output: 49
