问题
I am attempting to implement a very simple Trie in Java that supports 3 operations. I'd like it to have an insert method, a has method (ie is a certain word in the trie), and a toString method to return the trie in string form. I believe I have insertion working properly, but has and toString are proving to be difficult. Here's what I have so far.
The trie class.
public class CaseInsensitiveTrie implements SimpleTrie {
//root node
private TrieNode r;
public CaseInsensitiveTrie() {
r = new TrieNode();
}
public boolean has(String word) throws InvalidArgumentUosException {
return r.has(word);
}
public void insert(String word) throws InvalidArgumentUosException {
r.insert(word);
}
public String toString() {
return r.toString();
}
public static void main(String[] args) {
CaseInsensitiveTrie t = new CaseInsensitiveTrie();
System.out.println("Testing some strings");
t.insert("TEST");
t.insert("TATTER");
System.out.println(t.has("TEST"));
}
}
And the node class
public class TrieNode {
//make child nodes
private TrieNode[] c;
//flag for end of word
private boolean flag = false;
public TrieNode() {
c = new TrieNode[26]; //1 for each letter in alphabet
}
protected void insert(String word) {
int val = word.charAt(0) - 64;
//if the value of the child node at val is null, make a new node
//there to represent the letter
if (c[val] == null) {
c[val] = new TrieNode();
}
//if word length > 1, then word is not finished being added.
//otherwise, set the flag to true so we know a word ends there.
if (word.length() > 1) {
c[val].insert(word.substring(1));
} else {
c[val].flag = true;
}
}
public boolean has(String word) {
int val = word.charAt(0) - 64;
if (c[val]!=null && word.length()>1) {
c[val].has(word.substring(1));
} else if (c[val].flag==true && word.length()==1) {
return true;
}
return false;
}
public String toString() {
return "";
}
}
So basically, when creating a Trie, a TrieNode is created as the root with 26 children. When an insert is attempted, insert is called on that root node, which recursively creates a new node at the correct position, and continues until the word is complete. I believe that method is working properly.
My has function is very broken, because I have to have that return statement outside of the brackets for some reason. I cannot contain it within an else clause or the compiler complains. Other than that, I am thinking that method should work with some tweaks, but I cannot figure it out for the life of me.
toString is a beast I have tried to tackle, but nothing I throw at it works, so I will leave that be until I solve the has problem. If I get has working I may be able to figure a way to reformat it into a toString function.
The purpose of the int val = word.charAt(0) - 64; is because each string entered must be all caps (I will create a string formatting function to ensure this afterwards) so the first letter's int value - 64 will be it's position in the array. ie array index 0 is A, so A = 64, A - 64 = 0. B = 65, B - 64 = 1, and so on.
回答1:
Your has function should probably look like this:
if (c[val]!=null && word.length()>1) {
return c[val].has(word.substring(1)); //<-- Change is on this line
} else if (c[val].flag==true && word.length()==1) {
...etc
You perform the recursive call, but you really need to let that value propagate back out to the original caller.
回答2:
Maybe you can just use "Map c"instead of "TrieNode[] c", that would allow you to use this for all the types of characters uppercase/lowercase and even special characters and even would save you space ( allocating 26 character array at each character level )
回答3:
Here is my implementation: -
public class Tries {
class Node {
HashMap<Character, Node> children;
boolean end;
public Node(boolean b){
children = new HashMap<Character, Tries.Node>();
end = false;
}
}
private Node root;
public Tries(){
root = new Node(false);
}
public static void main(String args[]){
Tries tr = new Tries();
tr.add("dog");
tr.add("doggy");
System.out.println(tr.search("dogg"));
System.out.println(tr.search("doggy"));
}
private boolean search(String word) {
Node crawl = root;
int n = word.length();
for(int i=0;i<n;i++){
char ch = word.charAt(i);
if(crawl.children.get(ch) == null){
return false;
}
else {
crawl = crawl.children.get(ch);
if(i==n-1 && crawl.end == true){
return true;
}
}
}
return false;
}
private void add(String word) {
Node crawl = root;
int n = word.length();
for(int i=0;i<n;i++){
char ch = word.charAt(i);
if(crawl.children.containsKey(ch)){
crawl = crawl.children.get(ch);
}
else {
crawl.children.put(ch, new Node(false));
Node temp = crawl.children.get(ch);
if(i == n-1){
temp.end = true;
}
crawl = temp;
System.out.println(ch + " " + crawl.end);
}
}
}
}
回答4:
Here my implementation:
public class Tries {
private static class Leaf {
private Leaf(char c) {
this.c=c;
}
char c;
int counter = 1;
List<Leaf> leaves = new ArrayList<>(10);
}
private Leaf root = new Leaf('0');
public void add(String word) {
Leaf current = root;
Leaf newLeaf = null;
for (char c : word.toCharArray()) {
boolean found = false;
for (Leaf leaf : current.leaves) {
if (leaf.c == c) {
current = leaf;
current.counter++;
found=true;
break;
}
}
if (!found) {
newLeaf = new Leaf(c);
current.leaves.add(newLeaf);
current = newLeaf;
}
}
}
public int find(String partial) {
Leaf current = root;
for (char c : partial.toCharArray()) {
boolean found = false;
for (Leaf leaf : current.leaves) {
if (leaf.c == c) {
current=leaf;
found=true;
break;
}
}
if(!found) return 0;
}
return current.counter;
}
public boolean hasWord(String partial) {
return find(partial)>0;
}
}
回答5:
This Trie Map implements has (called get in this implementation) and toString
回答6:
Here is simple java implementation without using any other data structure
import java.util.ArrayList;
import java.util.List;
class Trie {
private static Node root = new Node(' ', false);
static int getIndex(char x) {
return ((int) x) - ((int) 'a');
}
static class Node {
char data;
boolean isLeaf;
Node[] children;
Node(char data, boolean leaf) {
this.data = data;
this.isLeaf = leaf;
this.children = new Node[26];
}
}
static void insert(String data, Node root) {
if (data == null || data.length() == 0) {
return;
}
Node child = root.children[getIndex(data.charAt(0))];
if (child == null) {
Node node = new Node(data.charAt(0), data.length() == 1);
root.children[getIndex(data.charAt(0))] = node;
if (data.length() > 1) {
insert(data.substring(1, data.length()), node);
}
} else {
if (data.length() == 1) {
child.isLeaf = true;
} else {
insert(data.substring(1, data.length()), child);
}
}
}
static boolean find(String data, Node root) {
if (data == null || data.length() == 0) {
return true;
}
char x = data.charAt(0);
//note that first node ie root is just dummy, it just holds important
Node node = root.children[getIndex(x)];
if (node == null) {
return false;
} else {
if (data.length() == 1) {
return node.isLeaf;
} else {
return find(data.substring(1, data.length()), node);
}
}
}
static boolean delete(String data, Node root) {
if (data == null || data.length() == 0) {
return false;
}
char x = data.charAt(0);
//note that first node ie root is just dummy, it just holds important
Node node = root.children[getIndex(x)];
if (node == null) {
return false;
} else {
if (data.length() == 1) {
node.isLeaf = false;
boolean allNull = true;
for (Node node1 : node.children) {
allNull = allNull && node1 == null;
}
return allNull;
} else {
boolean delete = delete(data.substring(1, data.length()), node);
if (delete) {
node.children[getIndex(x)] = null;
if(node.isLeaf){
return false;
}
boolean allNull = true;
for (Node node1 : node.children) {
allNull = allNull && node1 == null;
}
return allNull; }
}
}
return false;
}
private static List<String> strings = new ArrayList<>();
private static List<String> getAll() {
strings = new ArrayList<String>();
findAllDFS(root, "");
return strings;
}
private static void findAllDFS(Node node, String old) {
if (node != null) {
if (node.data != ' ') {
old = old + node.data;
}
if (node.isLeaf) {
strings.add(old);
}
for (Node node1 : node.children) {
findAllDFS(node1, old);
}
}
}
public static void main(String[] args) {
insert("abc", root);
insert("xyz", root);
insert("abcd", root);
insert("abcde", root);
delete("abcd", root);
/* System.out.println(find("abc", root));
System.out.println(find("abcd", root));
System.out.println(find("ab", root));
System.out.println(find("xyz", root));*/
System.out.println(getAll());
}
}
来源：https://stackoverflow.com/questions/2225540/trie-implementation