Given a string s, return whether s is a valid number.
For example, all the following are valid numbers: ["2", "0089", "-0.1", "+3.14", "4.", "-.9", "2e10", "-90E3", "3e7", "-6e-1", "53.5e93", "-123.456e789"], while the following are not valid numbers: ["abc", "1a", "1e", "e3", "99e2.5", "--6", "-+3", "95a54e53"].
Formally, a valid number is defined using one of the following definitions:
- An integer number followed by an optional exponent.
- A decimal number followed by an optional exponent.
An integer number is defined with an optional sign (- or +) followed by digits.
A decimal number is defined with an optional sign (- or +) followed by one of the following definitions:
2. Digits followed by a dot (.).
2. Digits followed by a dot (.) followed by digits.
2. A dot (.) followed by digits.
An exponent is defined with an exponent notation (e or E) followed by an integer number.
The digits are defined as one or more digits.