LeetCode: String to Integer (atoi)

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.

   int atoi(const char *str) {  
     if(!str)return 0;  
       
     int sign = 1, i = 0,prev;  
     long long res = 0;  
       
     while(str[i] == ' ' || str[i] == '\t')i++;  
       
     if(str[i] == '-'){sign = -1;i++;}  
     else if (str[i] == '+')i++;  
       
     while(str[i])  
     {  
       int x = str[i]-48;  
       if(x<0 || x>9 )return (res*sign);prev = res;  
       res = (res<<3) + (res<<1) + x;  
       if(res > 2147483647){if(sign == -1)return -2147483648; else return 2147483647;}   
       i++;  
     }  
     return ((int)res*sign);  
   }