LeetCode8 字符串转换整数 Python3

1、自己解法，一次遍历，找出符合条件的数字，转化

class Solution:
    def myAtoi(self, s: str) -> int:
        # 定义一个数组存数字？
        up = pow(2,31)-1
        low = -up-1
        slen = len(s)
        i,start,renum,sign = 0,-1,0,1
        restr = []
        if slen<1:
            return 0

        # 找到起始位置
        for c in s:
            if(c != ' '):
                if(c == '-'):
                    sign = -1
                    start = i+1
                    break
                elif(c == '+'):
                    sign = 1
                    start = i+1
                    break
                elif(c>='0' and c<='9'):
                    sign = 1 
                    start = i
                    break 
                else:
                    return 0
            i=i+1

        # 存入数组
        i=start
        while(i<slen):
            if(s[i]<'0' or s[i]>'9'):
                break
            else:
                restr.append(s[i])
                i = i+1    

        # 转化成数字
        for c in (restr):
            renum = renum*10 + int(c)-int('0')

        if(sign*renum > up):
            return up
        elif(sign*renum < low):
            return low
        else:
            return sign*renum
          

2、正则（现学一下）

class Solution:
    def myAtoi(self, s: str) -> int:
        UP = 2147483647    
        LOW = -2147483648

        s = s.lstrip() # 清除前面的空格
        temp = re.compile('^[\+\-]?\d+') # \+\- 或+或- ? 可有可无 \d 数字 +前面一个或多个 ^ 从头开始匹配
        num = temp.findall(s) # findall函数，找出匹配字符串
        num = int(*num) # 解包
        if(num>UP):
            return UP
        elif(num<LOW):
            return LOW
        else:
            return num

简单学习正则，学习lstrip函数，findall函数 和 解包

3、有限状态机（重点）

官方解答的图：

UP = 2147483647    
LOW = -2147483648

class Automata:
    def __init__(self):
        self.state="start"
        self.ans=0
        self.sign=1
        # 如果前面是start/sign/innum/end状态
        # 后面出现各种字符所对应的状态应该是...
        self.table={
            "start":["start","sign","innum","end"],
            "sign":["end","end","innum","end"],
            "innum":["end","end","innum","end"],
            "end":["end","end","end","end"]
        }
    def get_s(self,c):
        if c.isspace():
            return 0
        if c=='+' or c=='-':
            return 1
        if c.isdigit():
            return 2
        return 3
    def get(self,c):
        # 得到对应的状态
        self.state=self.table[self.state][self.get_s(c)]
        if self.state=="innum":
            self.ans = self.ans*10+int(c)
            if self.ans*self.sign > UP:
                self.ans=UP
            if self.ans*self.sign < LOW:
                self.ans=self.sign*LOW
        if self.state=="sign" and c=='-':
            self.sign=-1




class Solution:
    def myAtoi(self, s: str) -> int:

        auto=Automata()
        for c in s:
            auto.get(c)
            if auto.state=="end":
                break

        return auto.ans*auto.sign