Validate if a given string is numeric.

Some examples:

"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

细节实现题。

本题的功能与标准库中的strtod()功能类似。

{% codesnippet "./code/valid-number."+book.suffix, language=book.suffix %}{% endcodesnippet %}