Leetcode: String to Integer (atoi) (string)

PROBLEM:

https://leetcode.com/problems/string-to-integer-atoi/


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Basically the problem want to translate:
   "   +987778xw " 
into an integer.

Note:
1. limit check: INT_MAX, INT_MIN;
    the problem indicate the  INT_MAX (2147483647) or INT_MIN (-2147483648). the value 2147483647 is the limit of unsigned long long!! See below:

INT_MIN	Minimum value for an object of type int	-32767 (-215+1) or less*
INT_MAX	Maximum value for an object of type int	32767 (215-1) or greater*
UINT_MAX	Maximum value for an object of type unsigned int	65535 (216-1) or greater*
LONG_MIN	Minimum value for an object of type long int	-2147483647 (-231+1) or less*
LONG_MAX	Maximum value for an object of type long int	2147483647 (231-1) or greater*
ULONG_MAX	Maximum value for an object of type unsigned long int	4294967295 (232-1) or greater*

This confused me. 

2. 

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//code 12ms

class Solution {

public:

       int myAtoi(string str) {

              //pass the initial space

              int i = 0;

              unsigned long long res = 0;

              while (str[i] == ' ')i++;

              //parse the first non space char

              if (str[i] != '+' && str[i] != '-' && isdigit(str[i]) == 0)return 0;

              bool neg = false;

              if (str[i] == '+') { neg = false; i++; }

              else if (str[i] == '-') { neg = true; i++; }

              else neg = false;

              //parse the char followered by + -

              if (isdigit(str[i]) == 0)return 0;

              else {

                     while (isdigit(str[i])){

                           res = res * 10 + str[i] - '0';

                           if (res>INT_MAX)return neg ? INT_MIN : INT_MAX;

                           i++;

                     }

              }

              return neg ? -res : res;

       }

};