python leetcode 8. String to Integer (atoi)

题目并不难 但是你必须考虑到所有的异常情况

class Solution:
    def myAtoi(self, str):
        """
        :type str: str
        :rtype: int
        """
        if str=="":
            return 0
        f = 0 
        for i in range(len(str)):
            s=str[i]
            if s ==" ":
                continue
            elif s=="-":
                if i== len(str)-1 or not (str[i+1]>='0' and str[i+1]<='9'):
                    return 0
                if f==0:
                    f = -1
                else:
                    return 0 
            elif s=="+":
                if i== len(str)-1 or not (str[i+1]>='0' and str[i+1]<='9'):
                    return 0
                if f==0:
                    f=1
                else:
                    return 0 
            elif s>='0' and s<='9':
                break
            else:
                return 0
        res=''
        while i<len(str) and str[i]>='0' and str[i]<='9':
            res+=str[i]
            i+=1
        if res=="":
            return 0
        re = int(res)
        if f==-1:
            re=-re
        if re>2**31-1:
            return 2**31-1 
        
        if re<-2**31:
            return -2**31
        return re