leetcode: 93. Restore IP Addresses

# Given a string containing only digits, 
# restore it by returning all possible valid IP address combinations.
#
# For example:
# Given "25525511135",
#
# return ["255.255.11.135", "255.255.111.35"]. (Order does not matter)

class Solution():
    def restoreIpAddresses(self, s):
        ans = []
        self.helper(ans, s, 4, [])
        return ['.'.join(x) for x in ans]

    def helper(self, ans, s, k, temp):
        if len(s) > k*3:
            return
        if k == 0:
            ans.append(temp[:])
        else:
            for i in range(min(3,len(s)-k+1)):
                if i==2 and int(s[:3]) > 255 or i > 0 and s[0] == '0':
                    continue
                self.helper(ans, s[i+1:], k-1, temp+[s[:i+1]])


if __name__ == "__main__":
    assert Solution().restoreIpAddresses("0000") == ['0.0.0.0']