U面经Prepare: Web Address

题目是给你一堆域名，其中一些是另一些的parent，比如.com是.youku.com的parent，然后.youku.com是.service.youku.com的parent这样，然后再给你一个网址，让你在那堆域名中找到这个网址的parent里面最长的一个，然后再往前退一个返回。语言有点不好描述，举个栗子：

Domains:[
“.com”,
“.cn”
“.service.com”
“.net”
“.youku.net”
]
.
url: “yeah.hello.youku.net”

这里.net和.youku.net都是这个url的parent,其中最长的是.youku.net，再往前退一个是hello,所以返回“hello.youku.net”

虽然我觉得这道题用set倒着来就可以解决，但是看到一个Trie的做法也很不错

这里TrieNode.val不再是char, 而是String.  children array也变成了Map<String, TrieNode>

 public class LongestSubDomain {
     private class TrieNode {
         String str;
         Map<String, TrieNode> map;
         boolean isLeaf;
         public TrieNode(String str) {
             this.str = str;
             this.map = new HashMap<>();
             this.isLeaf = false;
         }
     }
     TrieNode root = new TrieNode("#");
     public static void main(String[] args) {
         LongestSubDomain lsd = new LongestSubDomain();
         String[] domains = {".com", ".cn", ".service.com", ".net", ".youku.net"};
         String url = "yeah.hello.youku.net";
         for (String str : domains) {
             lsd.insert(str);
         }
         String res = lsd.startWith(url);
         System.out.println(res);
     }
     public void insert(String domain) {
         String[] temp = domain.split("\\.");
         TrieNode node = root;
         for (int i = temp.length - 1; i >= 0; i--) {
             if (temp[i].length() == 0) continue;
             if (node.map.containsKey(temp[i])) {
                 node = node.map.get(temp[i]);
             } else {
                 TrieNode newNode = new TrieNode(temp[i]);
                 node.map.put(temp[i], newNode);
                 node = newNode;
             }
         }
         node.isLeaf = true;
     }
     public String startWith(String url) {
         String[] temp = url.split("\\.");
         TrieNode node = root;
         String res = "";
         int index = temp.length - 1;
         for (int i = temp.length - 1; i >= 0; i--) {
             if (temp[i].length() == 0) continue;
             if (node.map.containsKey(temp[i])) {
                 res = "." + temp[i] + res;
                 node = node.map.get(temp[i]);
             } else {
                 if (!node.isLeaf) {
                     res = "";
                 } else {
                     index = i;
                 }
                 break;
             }
         }
         return temp[index] + res;
     }
 }

我的做法：

 package uberOnsite;

 import java.util.*;

 public class LongestParent {
     public class TrieNode {
         String val;
         Map<String, TrieNode> children;
         boolean end;
         public TrieNode(String str) {
             val = str;
             children = new HashMap<String, TrieNode>();
             end = false;
         }
     }

         TrieNode root = new TrieNode("");

         public void insert(String str) {
             String[] arr = str.split("\\.");
             TrieNode node = root;
             for (int i=arr.length-; i>=; i--) {
                 if (arr[i].length() == ) continue;
                 if (!node.children.containsKey(arr[i])) {
                     node.children.put(arr[i], new TrieNode(arr[i]));
                 }
                 node = node.children.get(arr[i]);
             }
             node.end = true;
         }

         public String findLongest(String str) {
             String[] input = str.split("\\.");
             TrieNode node = root;
             StringBuffer res = new StringBuffer();
             for (int i=input.length-; i>=; i--) {
                 String cur = input[i];
                 if (node.children.containsKey(cur)) {
                     res.insert(, cur);
                     res.insert(, ".");
                     node = node.children.get(cur);
                 }
                 else {
                     res.insert(, cur);
                     return res.toString();
                 }
             }
             return "";
         }

     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         LongestParent sol = new LongestParent();
         String[] domains = {".com", ".cn", ".service.com", ".net", ".youku.net"};
         String url = "yeah.hello.youku.net";
         for (String each : domains) {
             sol.insert(each);
         }
         String res = sol.findLongest(url);
         System.out.println(res);
     }

 }