题目描述：

方法一：暴力法

class Solution:     def restoreIpAddresses(self, s: str) -> List[str]:         n = len(s)         res = [] # 判读是否满足ip的条件         def helper(tmp):             if not tmp or (tmp[0] == "0" and len(tmp) > 1) or int(tmp) > 255:                 return False             return True         # 三个循环,把数字分成四份         for i in range(3):             for j in range(i + 1, i + 4):                 for k in range(j + 1, j + 4):                     if i < n and j < n and k < n:                         tmp1 = s[:i + 1]                         tmp2 = s[i + 1:j + 1]                         tmp3 = s[j + 1:k + 1]                         tmp4 = s[k + 1:]                         # print(tmp1, tmp2, tmp3, tmp4)                         if all(map(helper, [tmp1, tmp2, tmp3, tmp4])):                             res.append(tmp1 + "." + tmp2 + "." + tmp3 + "." + tmp4)         return res

方法二;回溯

class Solution:     def restoreIpAddresses(self, s: str) -> List[str]:         res = []         n = len(s)         def backtrack(i, tmp, flag):             if i == n and flag == 0:                 res.append(tmp[:-1])                 return             if flag < 0:                 return             for j in range(i, i + 3):                if j < n:                     if i == j and s[j] == "0":                         backtrack(j + 1, tmp + s[j] + ".", flag - 1)                         break                     if 0 < int(s[i:j + 1]) <= 255:                         backtrack(j + 1, tmp + s[i:j + 1] + ".", flag - 1)         backtrack(0, "", 4)         return res