Sword finger offer (49): convert a string to an integer

solution 1： This problem should be considered comprehensively , Deal with outliers . For this subject , The key points to be noted are ：
· Whether the pointer is a null pointer and whether the string is an empty string ;
· Handling of signs in strings ;
· Whether the input value is a legal value , Less than or equal to ’9’, Greater than or equal to ’0’;
·int by 32 position , It is necessary to judge whether it overflows ;

class Solution:
    def StrToInt(self, s):
        # write code here
        if len(s) == 0:
            return 0
        else:
            minus = False
            flag = False
            if s[0] == '+':
                flag = True
            if s[0] == '-':
                flag = True
                minus = True
            begin = 0
            if flag:
                begin = 1
            num = 0
            minus = -1 if minus else 1
            for each in s[begin:]:
                if each >= '0' and each <= '9':
                    num = num*10 + minus*(ord(each) - ord('0'))
                else:
                    num = 0
                    break
            return num