# Time:  O(n^m) = O(3^4)
# Space: O(n * m) = O(3 * 4)
#
# 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:
    # @param s, a string
    # @return a list of strings
    def restoreIpAddresses(self, s):
        result = []
        self.restoreIpAddressesRecur(result, s, 0, "", 0)
        return result
        
    def restoreIpAddressesRecur(self, result, s, start, current, dots):
        # pruning to improve performance
        if (4 - dots) * 3 < len(s) - start or (4 - dots) > len(s) - start:
            return
        
        if start == len(s) and dots == 4:
            result.append(current[:-1])
        else:
            for i in xrange(start, start + 3):
                if len(s) > i and self.isValid(s[start:i + 1]):
                    current += s[start:i + 1] + '.'
                    self.restoreIpAddressesRecur(result, s, i + 1, current, dots + 1)
                    current = current[:-(i - start + 2)]
        
    def isValid(self, s):
        if len(s) == 0 or (s[0] == "0" and s != "0"):
            return False
        return int(s) < 256
    
if __name__ == "__main__":
    print Solution().restoreIpAddresses("0000")

# Time:  O(n^m) = O(3^4)
# Space: O(n * m) = O(3 * 4)
#
# 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:
    # @param s, a string
    # @return a list of strings
    def restoreIpAddresses(self, s):
        result = []
        self.restoreIpAddressesRecur(result, s, 0, "", 0)
        return result
        
    def restoreIpAddressesRecur(self, result, s, start, current, dots):
        # pruning to improve performance
        if (4 - dots) * 3 < len(s) - start or (4 - dots) > len(s) - start:
            return
        
        if start == len(s) and dots == 4:
            result.append(current[:-1])
        else:
            for i in xrange(start, start + 3):
                if len(s) > i and self.isValid(s[start:i + 1]):
                    current += s[start:i + 1] + '.'
                    self.restoreIpAddressesRecur(result, s, i + 1, current, dots + 1)
                    current = current[:-(i - start + 2)]
        
    def isValid(self, s):
        if len(s) == 0 or (s[0] == '0' and s != "0"):
            return False
        return int(s) < 256
    
if __name__ == "__main__":
    print Solution().restoreIpAddresses("0000")

Update restore-ip-addresses.py