Snapchat - IPFilter

这道题题意好难理解……

首先说一下IP，二进制表示的IP有32位，一般8位一分隔，转换成十进制，就是0.0.0.0 ~ 255.255.255.255

给你一些IP的规则，比如"7.7.7.7/8","123.2.67.3/8"，这个代表着比如7.7.7.7所对应的32位的表示，00000111*4，8表示取前8位，把所有以00000111开头的ip地址都看成一组，如果在这组内就被filter掉。

所以思路是：

1. 建立ipRule的trie，提供搜索的功能

2. 搜索

  1     public List<String> filter(List<String> rules, List<String> IPs) {
  2         if(rules == null || rules.size() == 0 || IPs == null || IPs.size() == 0) {
  3             return IPs;
  4         }
  5         Trie ruleTrie = new Trie();
  6         prepareRule(ruleTrie, rules);
  7         List<String> res = new ArrayList<String>();
  8         filterIPs(IPs, ruleTrie, res);
  9         return res;
 10     }
 11
 12
 13     private void prepareRule(Trie ruleTrie, List<String> rules) {
 14         for(String rule: rules) {
 15             StringBuilder sb = new StringBuilder();
 16             String[] parts = rule.split("/");
 17             int bits = Integer.parseInt(parts[1]);
 18             String[] ipBlocks = parts[0].split("\\.");
 19             for(String block: ipBlocks) {
 20                 String blockStr = Integer.toBinaryString(Integer.parseInt(block));
 21                 int leadingZeros = 8 - blockStr.length();
 22                 while(leadingZeros-- > 0) {
 23                     sb.append("0");
 24                 }
 25                 sb.append(blockStr);
 26             }
 27             String curRule = sb.toString().substring(0, bits);
 28             ruleTrie.insert(curRule);
 29         }
 30     }
 31
 32     private void filterIPs(List<String> IPs, Trie ruleTrie, List<String> res) {
 33         for(String ip: IPs) {
 34             StringBuilder sb = new StringBuilder();
 35             String[] ipBlocks = ip.split("\\.");
 36             for(String block: ipBlocks) {
 37                 String blockStr = Integer.toBinaryString(Integer.parseInt(block));
 38                 int leadingZeros = 8 - blockStr.length();
 39                 while(leadingZeros-- > 0) {
 40                     sb.append("0");
 41                 }
 42                 sb.append(blockStr);
 43             }
 44             String binaryIP = sb.toString();
 45             if(!ruleTrie.startsWith(binaryIP)) {
 46                 res.add(ip);
 47             }
 48         }
 49     }
 50
 51
 52     class TrieNode {
 53         char c;
 54         boolean isLeaf;
 55         TrieNode[] children;
 56
 57         public TrieNode() {
 58             children = new TrieNode[2];
 59             isLeaf = false;
 60         }
 61     }
 62
 63     class Trie {
 64         TrieNode root;
 65
 66         public Trie() {
 67             root = new TrieNode();
 68         }
 69
 70         public void insert(String s) {
 71             if(s == null || s.length() == 0) {
 72                 return;
 73             }
 74             TrieNode cur = root;
 75             for(int i = 0; i < s.length(); i++) {
 76                 char c = s.charAt(i);
 77                 int index = c - ‘0‘;
 78                 if(cur.children[index] == null) {
 79                     cur.children[index] = new TrieNode();
 80                 }
 81                 cur.children[index].c = c;
 82                 cur = cur.children[index];
 83             }
 84             cur.isLeaf = true;
 85         }
 86
 87         public boolean startsWith(String s) {
 88             TrieNode cur = root;
 89             for(int i = 0; i < s.length(); i++) {
 90                 char c = s.charAt(i);
 91                 int index = c - ‘0‘;
 92                 if(cur.isLeaf == true) {
 93                     return true;
 94                 }
 95                 if(cur.children[index] == null) {
 96                     return false;
 97                 }
 98                 cur = cur.children[index];
 99             }
100             return cur.isLeaf;
101         }
102     }
103
104     public static void main(String[] args) {
105         IPFilter sample = new IPFilter();
106         List<String> rules = new ArrayList<String>(Arrays.asList("7.7.7.7/8","123.2.67.3/8"));
107         List<String> IPs = new ArrayList<String>(Arrays.asList("23.2.4.1","7.3.4.1","5.1.2.3","123.2.67.3"));
108         List<String> res = sample.filter(rules, IPs);
109         System.out.println(res);
110     }