leetcode String to Integer (atoi)

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.

就是写一个atoi函数， 要注意很多要求， 我提交了n遍才被ac。。。。

1.判断空串
2.去除空格
3.第一个非空字符只能是+ , - , 或数字，找到数字后继续遍历直到遇到第一个非数字为止。
4.对于转换结果还要考虑溢出问题

code:

class Solution
{
   public:
     int myAtoi(string  str)
     { 
         if( str.length()==0 ) //判断是否为空串
           return 0;
         int i=0;
         int flag=0; //判断正负

         long long res=0;
         int max_int = 0x7fffffff;
         int min_int = 0x80000000;


          while(str[i] == ‘ ‘)
            i++;
          if(str[i] == ‘+‘)
            i++;
          else if(str[i] == ‘-‘)
          {
           flag=1;
           i++;
          }


         while(i!= str.length())
         {
          if( str[i]-‘0‘>=0 && str[i]-‘0‘ <=9 )
          {

              res = res*10 + (str[i]-‘0‘);
              if( res>max_int  )
                return (flag==0) ? max_int : min_int;

              i++;
           }
           else 
             break;
          }


         return (flag==0) ? res : -res ;
    }
};