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-1; i>=0; i--) {
                if (arr[i].length() == 0) 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-1; i>=0; i--) {
                String cur = input[i];
                if (node.children.containsKey(cur)) {
                    res.insert(0, cur);
                    res.insert(0, ".");
                    node = node.children.get(cur);
                }
                else {
                    res.insert(0, 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);
    }

}