[LeetCode] 93. Restore IP Addresses_Medium tag: backtracking

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

Example:

Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]

这个题目因为是要求所有的具体结果，所以需要用search， 考虑DFS，然后类似于[LeetCode] 131. Palindrome Partitioning_Medium tag: DFS, backtracking, Palindrome， 将它看成要加4刀使得符合“IP”的格式，每个要么是一位，如果2位，需要[10, 99], 如果3位，需要[100, 255]. 所以只是把palin() 改成了isValid()函数而已。

class Solution:
    def restoreIp(self, s):
　　　　 n, ans = len(s), []
　　　　 if n < 3 or n > 12: return ans
       def isValid(start, end, s):
            num = int(s[start: end + 1])
            if end > start + 2: return False
            elif end == start: return True
            elif end == start + 1: return  10 <= num <= 99
            else:
                return 100 <= num <= 255
        def helper(s, temp, ans, pos):
            if pos == len(s) and len(temp) == 4:
                ans.append('.'.join(temp))
            for i in range(pos, len(s)):
                if len(temp) < 4 and isValid(pos, i, s):
                    helper(s, temp + [s[pos: i + 1]], ans, i + 1)
        helper(s, [], ans, 0)
        return ans