表达式求值，包括括号的匹配问题、负数如何识别？2.2.3这样非法的数怎样识别？如何区别3+-2（非法）和3+（-2）（合法）以及3++2（非法）和3+（+2）（合法）-evaluates expressions, including parentheses matching, how to identify negative? 2.2.3 the number of such illegal identify what? What is the difference between 3 -2 (illegal) and 3 (2) (legal) and 3 (illegal) and 3 (2) (lawful)