Validate if a given string is numeric.

Some examples:
"0" => true
" 0.1 " => true
"abc" => false
"1 a" => false
"2e10" => true

Note: It is intended for the problem statement to be ambiguous. You should gather all requirements up front before implementing one.

class Solution {
public:
    bool isNum(const char *&s, bool flag)
    {
        bool result = false;
        bool neg = false;
        if (*s != '\0' )
        {
            if (*s == '-')
            {
                neg = true;
                ++s;
            }
            else if (*s == '+')
                ++s;
        }
        
        bool dot = false;
        while(*s != '\0')
        {
            if (*s >= '0' && *s <= '9')
            {
                result = true;
                ++s;
            }
            else if (*s == '.')
            {
                // e之后的数字不能为小数
                if (flag)
                    return false;
                if (dot == false)
                {
                    dot = true;
                    ++s;
                }
                else
                    return false;
            }
            else if (*s == 'e' || *s == ' ')
            {
                return result;
            }
            else
                return false;
        }
        return result;
    }
    
    bool isNumber(const char *s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (s == NULL) return false;
        bool neg = false;
        bool result = false;
        // 前置空白
        while(*s != '\0' && *s == ' ')
            ++s;
        
        // e之前的数字
        result = isNum(s, false);
        if (result == false)
            return false;
        
        // e之后
        if (*s == 'e')
        {
            ++s;
            result = isNum(s, true);
            if (result == false)
                return false;
        }
        
        // 后置空白
        while(*s != '\0' && *s == ' ')
            ++s;
        if (*s == '\0')
            return true;
        else
            return false;
    }
};