Leetcode: Restore IP Addresses

Question

Given a string containing only digits, restore it by returning all possible valid IP address combinations.

For example:
Given “25525511135”,

return [“255.255.11.135”, “255.255.111.35”]. (Order does not matter)

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My first try

class Solution(object):    def restoreIpAddresses(self, s):        """        :type s: str        :rtype: List[str]        """        res = []        if s=='':            return res        self.helper(s, 0, 3, '', res)        return res    def helper(self, s, start, part, item, res):        if part==0:            if len(s)-start>3:                return            else:                if self.check(s[start:]):                    res.append(item+s[start:])                return         for ind in range( start,min(start+3,len(s)-1) ):            # add '.'            if self.check(s[start:ind+1]):                self.helper(s, ind+1, part-1, item+s[start:ind+1]+'.', res)    def check(self, s):        if s=='':            return True        return True if int(s)>=0 and int(s)<256 else False

Error output:
0100110
0.100.1.10 is right, but 0.1.001.10 is not.

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My Solution

class Solution(object):    def restoreIpAddresses(self, s):        """        :type s: str        :rtype: List[str]        """        res = []        if s=='':            return res        self.helper(s, 0, 3, '', res)        return res    def helper(self, s, start, part, item, res):        if part==0:            if len(s)-start>3:                return            else:                if self.check(s[start:]):                    res.append(item+s[start:])                return         for ind in range( start,min(start+3,len(s)-1) ):   # the range is less than 3            if self.check(s[start:ind+1]):                self.helper(s, ind+1, part-1, item+s[start:ind+1]+'.', res)    def check(self, s):        if s=='':            return True        if len(s)>1:            for elem in s:                if elem!='0':                    break                else:                    return False        return True if int(s)>=0 and int(s)<256 else False