8.String to Integer (atoi) (INT; Overflow)

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.

思路：INT最大是2147483647，最小是-2147483648。所以要尤其注意-2147483648取相反数是得不到2147483648的。

class Solution {
public:
    int myAtoi(string str) {
       int ret = 0;
       int i = 0;
       bool pos = true;
       
       while(str[i] == ' ') i++; //neglect the spaces at the start
       if(str[i] == '+') i++;
       else if(str[i] == '-') {
           pos = false;
           i++;
       }
       
       while(str[i] != ''){
           if(str[i] > '9' || str[i] <'0') break; //if invalid digit, stop scanning and return the current result
           if(ret > 214748364 || (ret == 214748364 && str[i] - '0' > 7)){ //if overflow, return the extreme value
               if(pos) return INT_MAX;
               else return INT_MIN;
           }
           ret = ret * 10 + str[i] - '0';
           i++;
       }
       if(pos) return ret;
       else return (-ret);
    }
};