61. Valid Number

Given a string s, return whether s is a valid number. A valid number is composed of an optional sign, a decimal or an integer, and an optional exponent part. Formally: an optional '+' or '-'; then either a decimal (digits with an optional single '.' and at least one digit on some side) or an integer; then an optional exponent — 'e' or 'E' followed by an optional sign and one or more digits.