Implement a basic calculator to evaluate a simple expression string containing non-negative integers and operators '+', '-', '*', and '/'. The string can contain spaces. The integer division should truncate toward zero.
s = arithmetic expression string
integer result of the expression
Example 1:
Input:
s = "3+2*2"
Output:
7
Explanation:
Multiplication is evaluated before addition.
Example 2:
Input:
s = " 3/2 "
Output:
1
Explanation:
3/2 truncates toward zero to 1.