Restore IP Addresses

class Solution(object):    def restoreIpAddresses(self, s):        res = []        if len(s) > 12:            return res        def backtrack(i, dots, curIP):            if dots == 4 and i == len(s):                res.append(curIP[:-1])                return            if dots > 4:                return            for j in range(i, min(i+3, len(s))):                if (int(s[i: j + 1]) < 256) and (i == j or s[i] != "0"):                    backtrack(j + 1, dots + 1, curIP + s[i: j+1] + ".")        backtrack(0, 0, "")        return res