Leetcode: Restore IP Addresses @Python

class Solution:
    # @param s, a string
    # @return a list of strings
    def restoreIpAddresses(self, s):
        n=len(s)
        rst=[(0,[])]
        for i in range(4):
            newlist=[]
            for elem in rst:
                if (4-i)*3>=n-elem[0]:
                    for k in range(1,4):
                        if elem[0]+k<=n and (s[elem[0]]!='0' or k==1) and 0<=int(s[elem[0]:elem[0]+k])<=255:
                            newlist.append((k+elem[0],elem[1]+[s[elem[0]:elem[0]+k]]))
            rst=newlist
        res=[]    
        for elem in rst:
            if elem[0]==n:
                res.append('.'.join(elem[1]))
        return res