leetcode -- Restore IP Addresses -- 重点，dfs

https://leetcode.com/problems/restore-ip-addresses/

求 all possible solutions 肯定是dfs backtracking

参考http://www.cnblogs.com/zuoyuan/p/3768112.html
http://blog.csdn.net/linhuanmars/article/details/24683699

my code:

class Solution(object):

    def isValid(self, s):
        if s == '' or len(s) > 3:
            return False

        if len(s) > 1 and s[0] == '0':
            return False

        if 0<= int(s) <=255:
            return True    
        return False

    def dfs(self, s, slice_num, subres, res):
        if slice_num == 0 and len(s) == 0:
            res.append(subres[1:])
            return
        #elif len(s) == 0:#这个可以不要，不会出现第一段很大的情况
         # return
        else:
            for i in xrange(1, 4):#第一段不可能长度超过3
                if self.isValid(s[:i]) and i <= len(s):#注意这里要i<= len(s)这个条件，否则会出现重复解，否则也不能有s[i:]
                    self.dfs(s[i:], slice_num - 1, subres + '.' + s[:i], res)


    def restoreIpAddresses(self, s):
        """ :type s: str :rtype: List[str] """
        if len(s) > 12: return []
        res = []
        self.dfs(s, 4, '',res)
        return res