完美atoi，哈哈

 /*
 atoi算法，要求完美版
 有两种，一种是用longlong，一种是真用int
 “”
 "  "
 “-”
 “+”
 “  -23”
 “  +23”
 “12a"
 "abc"
 越界情况
 */
 #include <iostream>
 using namespace std;

 //方法一用 long long 取巧
 int atoi(const char * str,bool & flag)
 {
     long long result=;
     int tmp;
     int sign=;
     if(str==NULL)
     {
         flag=false;
         return ;
     }
     while(isspace(*str))
         str++;
     //if(*str=='\0')
     //{
     //    flag=false;
     //    return 0;
     //}
     if(*str=='+'||*str=='-')
     {
         if(*str=='-')
             sign=-;
         str++;
     }
     if(*str=='\0')                     //”    “和”+“ ，”-“的情况
     {
         flag=false;
         return ;
     }
     while(*str>=''&&*str<='')
     {
         tmp=*str-'';
         result=result*+tmp;
         if(sign==-)
         {
             if(result->INT_MAX)           //这里注意，如果是INT_MAX+1，系统或默认转换成int，导致越界，所以用result-1，result是longlong不会越界
             {
                 result=INT_MAX+;
                 flag=false;
                 break;
             }
         }
         else if(result>INT_MAX)
         {
             result=INT_MAX;
             flag=false;
             break;
         }
         str++;
     }
     if(*str=='\0')
         flag=true;
     else
         flag=false;
     return sign*result;
 }

 //方法二 用int
 int atoi2(const char * str ,bool & flag)
 {
     int result=;
     int sign=;
     int tmp;
     int MAX_DIV=INT_MAX/;
     int MAX_R=INT_MAX%;
     int MIN_R=MAX_R+;   //或者为MIN_R=-(INT_MIN%10);    //如果用(INT_MAX+1)%10就错了，直接加的时候越界
     int MIN_DIV=(INT_MAX)/;        

     //july用的是max=(int)((unsigned)~0>>1);
     //min_r=(int)(((unsigned)~0>>1+1)%10);
     if(str==NULL)
     {
         flag=false;
         return result;
     }
     while(isspace(*str))
         str++;
     if(*str=='+'||*str=='-')
     {
         if(*str=='-')
             sign=-;
         str++;
     }
     if(*str=='\0')
     {
         flag=false;
         return result;
     }
     while(*str>=''&&*str<='')
     {
         int tmp=*str-'';
         if(sign==)
         {
             if(result>MAX_DIV||(result==MAX_DIV&&tmp>MAX_R))
             {
                 result=INT_MAX;
                 flag=false;
                 return result;
             }
         }
         else if(result>MIN_DIV||(result==MIN_DIV&&tmp>MIN_R))
         {
             result=INT_MIN;                                               //result直接为最小-2147483648，直接返回了
             flag=false;
             return result;
         }
         result=result*+tmp;
         str++;
     }
     if(*str=='\0')
         flag=true;
     else
         flag=false;
     return sign*result;
 }

 int main()
 {
     char * str[]={""," ","abc","","  -1234","   +34","  12bc","+","-","+++","","","+2147483648","-2147483648","-2147483649","+12123131312432342","   -122323434534534654"};
     //cout<<INT_MAX<<endl;
     int n=;
     bool flag;
     for(int i=;i<n;i++)
     {
         int tmp=atoi2(str[i],flag);
         if(tmp==)
         {
             if(flag==false)
                 cout<<str[i]<<" 错误数据转换为： "<<tmp<<endl;
             else
                 cout<<str[i]<<" 转换为: "<<tmp<<endl;
         }
         else
         {
         cout<<str[i]<<" 转换为: "<<tmp<<endl;
         }
     }
     system("pause");
 }