65. Valid Number

A valid number can be split up into these components (in order):

1. A decimal number or an integer.
2. (Optional) An ‘e’ or ‘E’, followed by an integer.

A decimal number can be split up into these components (in order):

1. (Optional) A sign character (either ‘+’ or ‘-‘).
2. One of the following formats:

1. One or more digits, followed by a dot ‘.’.
2. One or more digits, followed by a dot ‘.’, followed by one or more digits.
3. A dot ‘.’, followed by one or more digits.

An integer can be split up into these components (in order):

1. (Optional) A sign character (either ‘+’ or ‘-‘).
2. One or more digits.

For example, all the following are valid numbers: [“2”, “0089”, “-0.1”, “+3.14”, “4.”, “-.9”, “2e10”, “-90E3”, “3e+7”, “+6e-1”, “53.5e93”, “-123.456e789”], while the following are not valid numbers: [“abc”, “1a”, “1e”, “e3”, “99e2.5”, “—6”, “-+3”, “95a54e53”].

Given a string s, return true if s is a valid number.

class Solution {
public:
    bool isNumber(string s) {
        bool sign = s[0] == '+' or s[0] == '-', e = false, dot = false, digit = false;
        for(int i = sign; i < s.length(); i++) {
            if(s[i] == '+' or s[i] == '-') {
                if(sign) return false;
                if(!e and i != 0) return false;
                if(e and digit) return false;
                sign = true;
            } else if(isdigit(s[i])) {
                digit = true;
            } else if(s[i] == 'e' or s[i] == 'E') {
                if(!digit) return false;
                if(e) return false;
                sign = false;
                digit = false;
                e = true;
            } else if(s[i] == '.') {
                if(e) return false;
                if(dot) return false;
                dot = true;
            } else return false;
        }
        return digit;
    }
};