leetcode-93-Restore IP Addresses

题目描述：Given a string containing only digits, restore it by returning all possible valid IP address combinations.

Example:

Input: "25525511135"
Output: ["255.255.11.135", "255.255.111.35"]

题目理解：

 将一段字符str广度搜索截取，分别有n种组合形式，添加限制条件，过滤掉不适合的组合元素。 长度，大小，首字母

应用：如果进行字符串的子元素组合穷举，可以应用。

  所有的for循环，利用到前一个状态，都可以理解为动态规划的一种分支

import copy
class Solution:
    def restoreIpAddresses(self, s):
        """
        :type s: str
        :rtype: List[str]
        """
        lens=[1,2,3]
        combinations_all=[]
        combinations_return=[]
        def bfs(len_cur,s_cur,combinations_in):
            for len_iter in lens:
                combinations_in_copied=copy.deepcopy(combinations_in)
                len_left_over=len_cur-len_iter
                id_cur=s_cur[:len_iter]
                if id_cur and int(id_cur)>255:
                    return
                # print(id_cur)
                # combinations_in.append(id_cur)
                combinations_in_copied.append(id_cur)
                if len(combinations_in_copied)<=4:
                    if len_left_over == 0 and len(combinations_in_copied)==4:
                        combinations_in_copied=list(filter(lambda x:not (len(x)>1 and x.startswith('0')),combinations_in_copied))
                        if len(combinations_in_copied)==4:
                            # print('combinations_in_copied==>',combinations_in_copied)
                            combinations_in_copied=[str(elem_iter) for elem_iter in combinations_in_copied]
                            elem='.'.join(combinations_in_copied)
                            combinations_return.append(elem)
                        # combinations_all.append(combinations_in_copied)
                        return
                    elif len_left_over<0:
                        return
                    else:
                        bfs(len_left_over,s_cur[len_iter:],combinations_in_copied)
                else:
                    return
        bfs(len(s),s,combinations_all)
        # print(combinations_return)
        return combinations_return