[Leetcode][python]Restore IP Addresses/复原IP地址

class Solution(object):
    def restoreIpAddresses(self, s):
        """
        :type s: str
        :rtype: List[str]
        """
        result = []
        self._restoreIpAddresses(0, s, [], result)
        return result

    def _restoreIpAddresses(self, length, s, ips, result):
        if not s:
            if length == 4:
                result.append('.'.join(ips))  # 以.分隔作为字符串返回
            return
        if length == 4:  # 分了4段，结束
            return

        # 取一位
        self._restoreIpAddresses(length + 1, s[1:], ips + [s[:1]], result)

        # 若要取2位及以上，要确保目前的第一位不能为0
        if s[0] != '0':  
            if len(s) >= 2:
                self._restoreIpAddresses(length + 1, s[2:], ips + [s[:2]], result)
            if len(s) >= 3 and int(s[:3]) <= 255: # 若要取3位，则要保证小于255
                self._restoreIpAddresses(length + 1, s[3:], ips + [s[:3]], result)