Leetcode008. String to Integer (atoi)

/* improvement on dealing  with overflow accroding to this:
 * https://discuss.leetcode.com/topic/57452/c-solution-beats-100
 * 
 * be careful about the INT_MAX and INT_MIN 
 * this atoi is not thinking about special char like dot and so on.
 */

class Solution {
public:
    int myAtoi(string str) {
       int i,nega_sign=1;
       long long int re=0;  //long long re to avoid overflow 
       for(i=0;i<str.size();i++)       //skip to all the blank in the front of the string
       {
           if(str[i]!=' ')break;
       }
       if(str[i]=='+')i++;             // deal with signature
       else if(str[i]=='-'){ nega_sign=-1;i++;}
       while(i<str.size()&&isdigit(str[i]))  
/*when the string is over or the current char is not a valid integral number,no more conversion will be performed.
*/
       {
               re=re*10+str[i]-'0';
               if(nega_sign==-1&& -1*re<=INT_MIN)return INT_MIN;      //overflow
               else if (nega_sign==1&&re>=INT_MAX)return INT_MAX;
               i++;
       }
       return re*nega_sign;
    }
};