[LeetCode] 65. Valid Number

There is some duplicates between function isInteger and isDecimal. But, in this time, I won;t be handle it.

class Solution {
public:
    static inline bool isDigit(char ch) {
        return '0' <= ch && ch <= '9';
    }

    bool isInteger(string s) {
        int i = 0, len = s.length();
        bool digit = false;
        if (len <= 0) { return false; }
        if (s[0] == '+' || s[0] == '-') { i++; }
        for(; i < len; i++) {
            if (!isDigit(s[i])) { return false; }
            digit = true;
        }
        return digit;
    }

    bool isDecimal(string s) {
        int i = 0, len = s.length();
        bool digit = false, dot = false;
        if (len <= 0) { return false; }
        if (s[0] == '+' || s[0] == '-') { i++; }
        for(; i < len; i++) {
            if (s[i] == '.') {
                if (dot) { return false; }
                else { dot = true; continue; }
            }
            if (!isDigit(s[i])) { return false; }
            digit = true;
        }
        return digit;
    }

    bool isNumber(string s) {
        int exponent = s.find('e');
        if (exponent == -1) { exponent = s.find('E'); }
        if (exponent == -1) { return isDecimal(s); }
        return isDecimal(s.substr(0, exponent)) && isInteger(s.substr(exponent+1));
    }
};