表示数值的字符串

参考剑指offer

class Solution {
    public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param str string字符串 
     * @return bool布尔型
     */
    bool scanUnsignedInteger(string str, int &index){

        int before = index;

        while(str[index] != '\0' && str[index] >= '0' && str[index] <= '9')
            index++;

        return index > before;
    }

    bool scanInteger(string str, int &index)
    {
        if(str[index] == '+' || str[index] == '-'){
            index++;
        }

        return scanUnsignedInteger(str, index);
    }

    bool isNumeric(string str) {
        // write code here
        int str_len = str.length();
        if(str_len <= 0)
            return false;
        int index = 0;
        bool numeric = scanInteger(str, index);
        if(str[index] == '.'){
            ++index;
            numeric = scanUnsignedInteger(str, index) || numeric;
        }
        if(str[index] == 'e' || str[index] ==  'E'){
            ++index;
            numeric = numeric && scanInteger(str, index);
        }
        return numeric&&str[index] == '\0';
    }
};