LC93 Restore IP Addresses

class Solution:
    """
    step1, set limit for 4 level recursion and stop; check if all string are used, if yes, push into result.  otherwise ignore.  return current path 
    step2, set substring length limit: 1-3.  and discuss number restrictions based on length variety. (len== 1:  no restriction; len ==2: first number not '0'; len == 3 first num not '1' and total number <=255)
    step3, when current substring meets desired standard, make the rest string go to next recursion.
    """
    def restoreIpAddresses(self, s: str) -> List[str]:
        res = []
        def dfs(ss,level,path,res):
            if level == 4:
                if not ss:
 #                   print(path[-1])
                    res.append(path[:-1])# remove last '.'
                return  #!!! be careful the position not algined with res 
            # @i:  length of substring at each segment
            # substring length max is 3
            for i in range(1,4):
                if i <= len(ss):  #!!!! important, make sure i is not overflow
                    #sub = s[:i]
                    if i == 1:
                        dfs(ss[i:],level+1,path+ss[:i]+'.',res)
                    elif i == 2 and ss[0]!='0':
                        dfs(ss[i:],level+1,path+ss[:i]+'.',res)
                    elif i == 3 and ss[0]!='0' and int(ss[:3]) <= 255:
                        dfs(ss[i:],level+1,path+ss[:i]+'.',res)
        dfs(s,0,"",res)
        return res