Basic Calculator II Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:

"3+2*2" = 7 " 3/2 " = 1 " 3+5 / 2 " = 5

Note: Do not use the eval built-in library function.

设计一个计算器，输入有效，并且只含有+, -, *, /，空格，全是非负整数