复原IP地址

题目描述

给定一个只包含数字的字符串，复原它并返回所有可能的 IP 地址格式。

示例:

输入: “25525511135”
输出: [“255.255.11.135”, “255.255.111.35”]

解题思路

DFS。搭配较多的剪枝可加快速度。

代码

Python 3.6

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class Solution:     def restoreIpAddresses(self, s: str) -> List[str]:         length = len(s)         if length < 4 or length > 12:             return []         res = []         self.__dfs(s, 0, length, [], res)         return res           def __dfs(self, s, start, length, path, res):         blank = 4 - len(path)         if blank == 0:             if start >= length:                 res.append(".".join(path.copy()))             return         if (length - start < blank) or (length - start > 3 * blank):             # 剩下的数字比空还少 或 比3*空还多             return         for i in range(1, 4):             end = start + i             if end > length or int(s[start:end]) > 255 or (i > 1 and s[start:end][0] == "0"):                 # 不合法：超过255、非单个数字且首位为0                 break             path.append(s[start:end])             self.__dfs(s, end, length, path, res)             path.pop()