I have code for prefix tree (Trie) like this:
public class Trie<V> {
Entry<V> entry;
char key;
Map<Character, Trie<V>> childrens;
public Trie() {
this.childrens = new HashMap<Character, Trie<V>>(10);
entry = new Entry<V>();
}
/** non-public, used by _put() */
Trie(char key) {
this.childrens = new HashMap<Character, Trie<V>>(10);
this.key = key;
entry = new Entry<V>();
}
public void put(String key, V value) {
_put(new StringBuffer(key), new StringBuffer(""), value);
}
void _put(StringBuffer remainder, StringBuffer prefix, V value) {
if (remainder.length() > 0) {
char keyElement = remainder.charAt(0);
Trie<V> t = null;
try {
t = childrens.get(keyElement);
} catch (IndexOutOfBoundsException e) {
}
if (t == null) {
t = new Trie<V>(keyElement);
childrens.put(keyElement, t);
}
prefix.append(remainder.charAt(0));
t._put(remainder.deleteCharAt(0), prefix, value);
} else {
this.entry.value = value;
this.entry.prefix = prefix.toString();
}
}
/**
* Retrieves element from prefix table matching as a prefix to provided key.
* E.g. is key is "abcde" and prefix table has node "ab" then this call will
* return "ab"
*
* @param key
* a string which starts with prefix to be searched in the table
* (e.g. phone number)
* @return an Object assosiated with matching prefix (i.e if key is a phone
* number it may return a corresponding country name)
*/
public V get(String key) {
return _get(new StringBuffer(key), 0);
}
/**
* Returns true if key has matching prefix in the table
*/
public boolean hasPrefix(String key) {
return ((this.get(key) != null) ? true : false);
}
V _get(StringBuffer key, int level) {
if (key.length() > 0) {
Trie<V> t = childrens.get(key.charAt(0));
if (t != null) {
return t._get(key.deleteCharAt(0), ++level);
} else {
return (level > 0) ? entry.value : null;
}
} else {
return entry.value;
}
}
@Override
public String toString() {
return "Trie [entry=" + entry + ", key=" + key + ", childrens="
+ childrens + "]";
}
static public class Entry<V> {
String prefix;
V value;
public Entry() {
}
public Entry(String p, V v) {
prefix = p;
value = v;
}
public String prefix() {
return prefix;
}
public V value() {
return value;
}
@Override
public String toString() {
return "Entry [prefix=" + prefix + ", value=" + value + "]";
}
}
}
Insertion goes like this:
private static Trie<String> trie = new Trie<>();
trie.put("7", "Some country");
trie.put("77", "Some other country");
trie.put("745", "Entirely different place");
Searching will go like this:
String result = trie.get("746878788");
System.out.println(result);
This search will result null because there is no value for 74.
My question is: how can I modify _get method in Trie class so that it will remember last value that is not null. So when it ends up in 74, then it will remember that 7 had some value "Some country", so it will return that instead of null.
Any ideas how to solve this?
Any help is appreciated!
First of all, I modified the toString() method of the Trie to get some better debug information. The only important rows to achieve what you are asking are these in _get method:
V result = t._get(key.deleteCharAt(0), ++level);
return result == null ? entry.value : result;
The trie now prefers current entry's value, if subentry's value is null.
The whole code modified:
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
public class Trie<V> {
Entry<V> entry;
char key;
Map<Character, Trie<V>> children;
public Trie() {
this.children = new HashMap<Character, Trie<V>>(10);
entry = new Entry<V>();
}
/** non-public, used by _put() */
Trie(char key) {
this.children = new HashMap<Character, Trie<V>>(10);
this.key = key;
entry = new Entry<V>();
}
public void put(String key, V value) {
_put(new StringBuffer(key), new StringBuffer(""), value);
}
void _put(StringBuffer remainder, StringBuffer prefix, V value) {
if (remainder.length() > 0) {
char keyElement = remainder.charAt(0);
Trie<V> t = null;
try {
t = children.get(keyElement);
} catch (IndexOutOfBoundsException e) {
}
if (t == null) {
t = new Trie<V>(keyElement);
children.put(keyElement, t);
}
prefix.append(remainder.charAt(0));
t._put(remainder.deleteCharAt(0), prefix, value);
} else {
this.entry.value = value;
this.entry.prefix = prefix.toString();
}
}
/**
* Retrieves element from prefix table matching as a prefix to provided key.
* E.g. is key is "abcde" and prefix table has node "ab" then this call will
* return "ab"
*
* @param key
* a string which starts with prefix to be searched in the table
* (e.g. phone number)
* @return an Object assosiated with matching prefix (i.e if key is a phone
* number it may return a corresponding country name)
*/
public V get(String key) {
return _get(new StringBuffer(key), 0);
}
/**
* Returns true if key has matching prefix in the table
*/
public boolean hasPrefix(String key) {
return ((this.get(key) != null) ? true : false);
}
V _get(StringBuffer key, int level) {
if (key.length() > 0) {
Trie<V> t = children.get(key.charAt(0));
if (t != null) {
V result = t._get(key.deleteCharAt(0), ++level);
return result == null ? entry.value : result;
} else {
return (level > 0) ? entry.value : null;
}
} else {
return entry.value;
}
}
@Override
public String toString() {
Iterator<Character> it = children.keySet().iterator();
StringBuffer childs = new StringBuffer();
while (it.hasNext()) {
Character key = it.next();
childs.append(String.format("\n%s\n",
// adding a tab to the beginning of every line to create a visual tree
String.format("%s: %s", key, children.get(key)).toString().replaceAll("(?m)(^)", "\t")));
}
return String.format("Trie [entry=%s, children=%s]", entry, childs);
}
static public class Entry<V> {
String prefix;
V value;
public Entry() {
}
public Entry(String p, V v) {
prefix = p;
value = v;
}
public String prefix() {
return prefix;
}
public V value() {
return value;
}
@Override
public String toString() {
return "Entry [prefix=" + prefix + ", value=" + value + "]";
}
}
}