有效Valid Number .

问题：给出一个字符串，判断它是否是一个有效的数字。

什么叫有效的数字呢？

整数 ，小数 "2.5"，正数 "+25"，负数"-25"，科学计数法"-2e1+0"。

特殊用例：

“.5”， “5.” 认为是有效数字。

无效用例：

“.", "e25", "25e", "2e5e5", "2.5.5", "2e2.5"

输入检查：左右两端出现空格是允许的，内部不能有空格。

class Solution {
public:
    bool isNumber(const char *s) {
        if(s == NULL)
            return false;

        int ex, dot;
        ex = dot = 0; // only once

        while(*s == ' ') // space
            s++;

        if(*s == '+' || *s == '-') // sign
            s++;

        if(isNum(*s)) // first char is numeric
            s++;
        else if(*s == '.') // if first char is '.'
        {
            s++;
            dot = 1;
            if(isNum(*s)) // then second char has to be numeric
                s++;
            else
                return false;
        }
        else
            return false;

        while(*s != '\0')
        {
            if(*s == '.')
            {
                if(ex == 1 || dot == 1)
                    return false;
                dot = 1;
            }
            else if(*s == 'e')
            {
                if(ex == 1)
                    return false;
                ex = 1;
                if(*(s+1) == '+' || *(s+1) == '-') //sign for exponent
                    s++;
                if(!isNum(*(s+1))) // numeric after 'e'
                    return false;
            }
            else if(*s == ' ')  // space has to be on the right side
            {
                break;
            }
            else if(!isNum(*s)) //other letter
            {
                return false;
            }

            s++;
        }
        while(*s == ' ') // space
            s++;
        if(*s == '\0')
            return true;
        else
            return false;
    }

    bool isNum(const char c)
    {
        return (c >= '0' && c <= '9'); // 0<=c<=9
    }
};

class Solution {
public:
    bool isNumber(const char *s) {
        if(s == NULL)
            return false;
            
        int ex, dot;
        ex = dot = 0; // only once
        
        while(*s == ' ') // space
            s++;
            
        if(*s == '+' || *s == '-') // sign
            s++;
            
        if(isNum(*s)) // first char is numeric
            s++;
        else if(*s == '.') // if first char is '.'
        {
            s++;
            dot = 1;
            if(isNum(*s)) // then second char has to be numeric
                s++;
            else
                return false;
        }
        else
            return false;
        
        while(*s != '\0')
        {
            if(*s == '.')
            {
                if(ex == 1 || dot == 1)
                    return false;
                dot = 1;
            }
            else if(*s == 'e')
            {
                if(ex == 1)
                    return false;
                ex = 1;
                if(*(s+1) == '+' || *(s+1) == '-') //sign for exponent
                    s++;
                if(!isNum(*(s+1))) // numeric after 'e' 
                    return false;
            }
            else if(*s == ' ')  // space has to be on the right side
            {
                break;
            }
            else if(!isNum(*s)) //other letter
            {
                return false;
            }
            
            s++;
        }
        while(*s == ' ') // space
            s++;
        if(*s == '\0')
            return true;
        else
            return false;
    }
    
    bool isNum(const char c)
    {
        return (c >= '0' && c <= '9'); // 0<=c<=9
    }
};