Given a string s which represents an expression, evaluate this expression and
return its value.
The integer division should truncate toward zero.
You may assume that the given expression is always valid. All intermediate
results will be in the range of [-231, 231 - 1].
Note: You are not allowed to use any built-in function which evaluates
strings as mathematical expressions, such as eval().
干你娘 好难喔
还要考虑先乘除后加减
好麻烦