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(object):
    def restoreIpAddresses(self, s):
        """
        :type s: str
        :rtype: List[str]
        """
        ans = []
        n = len(s)
        def isValid(num):
            if len(num) == 1:
                return True
            if len(num) > 1 and num[0] != "0" and int(num) <= 255:
                return True
            return False
            
        for i in range(0, min(3, n - 3)):
            a = s[:i+1]
            if not isValid(a):
                break
            for j in range(i + 1, min(i + 4, n - 2)):
                b = s[i+1:j+1]
                if not isValid(b):
                    break
                for k in range(j + 1, min(j + 4, n - 1)):
                    c = s[j+1:k+1]
                    d = s[k+1:]
                    if not isValid(c):
                        break
                    if not isValid(d):
                        continue
                    ans.append("{}.{}.{}.{}".format(a, b, c, d))
        return ans