String to Integer (atoi)

思路：

获取的知识点：0对应的ASCII码：48，一般判断时，需要用 ‘0’  ‘9’来去表示字符

这个题目需要自己考虑各种可能的输入，并加以处理。

1. discards all leading whitespaces  数字前的空格
2. sign of the number                        数字的符号
3. overflow                                        int型整数溢出
4. invalid input                                   输入是否合法，不合法就输出0；

题目：

Implement atoi to convert a string to an integer.

Hint: Carefully consider all possible input cases. If you want a challenge, please do not see below and ask yourself what are the possible input cases.

Notes: It is intended for this problem to be specified vaguely (ie, no given input specs). You are responsible to gather all the input requirements up front.

Update (2015-02-10):
The signature of the C++ function had been updated. If you still see your function signature accepts a const char * argument, please click the reload button  to reset your code definition.

spoilers alert... click to show requirements for atoi.

class Solution {public:    int myAtoi(string str) {        int len=str.size();        int i=0;        int sign=1;        long long ans=0;                if(len==0) return 0;        while(str[i]==' ') i++;        if(str[i]=='-')         {            sign=-1;            i++;        }        else if(str[i]=='+')        {            i++;        }        while(str[i]>='0'&&str[i]<='9'&&i<len)        {            ans=((int)str[i]-48)+ans*10;            if(ans*sign>INT_MAX) return INT_MAX;            if(ans*sign<INT_MIN) return INT_MIN;            i++;        }        return ans*sign;    }};