Implement a basic calculator to evaluate a simple expression string.
The expression string contains only non-negative integers,
/ operators and empty spaces
You may assume that the given expression is always valid.
"3+2*2" = 7 " 3/2 " = 1 " 3+5 / 2 " = 5
Note: Do not use the
eval built-in library function.
Special thanks to @ts for adding this problem and creating all test cases.