Implement a basic calculator to evaluate a simple expression string.
The expression string contains only non-negative integers, <code>+</code>, <code>-</code>, <code>*</code>, <code>/</code> operators and empty spaces <code></code>. The integer division should truncate toward zero.
You may assume that the given expression is always valid.
Some examples:
Note: Do not use the <code>eval</code> built-in library function.
Credits: