Trie

Trie/.gitignore

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+# Covers JetBrains IDEs: IntelliJ, RubyMine, PhpStorm, AppCode, PyCharm, CLion, Android Studio and Webstorm
+# Reference: https://intellij-support.jetbrains.com/hc/en-us/articles/206544839
+
+# User-specific stuff:
+.idea/**/workspace.xml
+.idea/**/tasks.xml
+.idea/dictionaries
+
+# Sensitive or high-churn files:
+.idea/**/dataSources/
+.idea/**/dataSources.ids
+.idea/**/dataSources.xml
+.idea/**/dataSources.local.xml
+.idea/**/sqlDataSources.xml
+.idea/**/dynamic.xml
+.idea/**/uiDesigner.xml
+
+# Gradle:
+.idea/**/gradle.xml
+.idea/**/libraries
+
+# CMake
+cmake-build-debug/
+
+# Mongo Explorer plugin:
+.idea/**/mongoSettings.xml
+
+## File-based project format:
+*.iws
+
+## Plugin-specific files:
+
+# IntelliJ
+out/
+
+# mpeltonen/sbt-idea plugin
+.idea_modules/
+
+# JIRA plugin
+atlassian-ide-plugin.xml
+
+# Cursive Clojure plugin
+.idea/replstate.xml
+
+# Crashlytics plugin (for Android Studio and IntelliJ)
+com_crashlytics_export_strings.xml
+crashlytics.properties
+crashlytics-build.properties
+fabric.properties

Trie/Trie.iml

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+<?xml version="1.0" encoding="UTF-8"?>
+<module org.jetbrains.idea.maven.project.MavenProjectsManager.isMavenModule="true" type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" LANGUAGE_LEVEL="JDK_1_5">
+    <output url="file://$MODULE_DIR$/target/classes" />
+    <output-test url="file://$MODULE_DIR$/target/test-classes" />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$/src/main/java" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/src/main/resources" type="java-resource" />
+      <sourceFolder url="file://$MODULE_DIR$/src/test/java" isTestSource="true" />
+      <excludeFolder url="file://$MODULE_DIR$/target" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+    <orderEntry type="library" name="Maven: junit:junit:4.0" level="project" />
+  </component>
+</module>

Trie/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>ru.spbau.mit.kazakov</groupId>
+    <artifactId>Trie</artifactId>
+    <version>1.0-SNAPSHOT</version>
+    <packaging>jar</packaging>
+
+    <dependencies>
+        <dependency>
+            <groupId>junit</groupId>
+            <artifactId>junit</artifactId>
+            <version>4.0</version>
+        </dependency>
+    </dependencies>
+
+</project>

Trie/src/main/java/ru/spbau/mit/kazakov/Trie/Trie.java

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+package ru.spbau.mit.kazakov.Trie;
+
+import java.io.*;
+import java.util.Hashtable;
+
+/**
+ * Prefix tree for storing strings.
+ */
+public class Trie implements Serializable {
+    private Node root = new Node();
+
+    /**
+     * Checks if there is specified string in trie.
+     *
+     * @param element a string to check
+     * @return true if there is specified string in trie, and false otherwise
+     */
+    public boolean contains(String element) {
+        Node current = root;
+
+        for (int i = 0; i < element.length(); i++) {
+            if (!current.edges.containsKey(element.charAt(i))) {
+                return false;
+            }
+            current = current.edges.get(element.charAt(i));
+        }
+
+        return current.isTerminal;
+    }
+
+    /**
+     * Adds specified string to trie.
+     *
+     * @param element a string to add
+     * @return true if there was no specified string in trie, and false otherwise
+     */
+    public boolean add(String element) {
+        if (contains(element)) {
+            return false;
+        }
+
+        Node current = root;
+        for (int i = 0; i < element.length(); i++) {
+            current.numOfContinuations++;
+            if (!current.edges.containsKey(element.charAt(i))) {
+                current.edges.put(element.charAt(i), new Node());
+            }
+            current = current.edges.get(element.charAt(i));
+        }
+        current.numOfContinuations++;
+        current.isTerminal = true;
+
+        return true;
+    }
+
+    /**
+     * Counts how many strings in trie starts with specified string.
+     */
+    public int howManyStartsWithPrefix(String prefix) {
+        Node current = root;
+
+        for (int i = 0; i < prefix.length(); i++) {
+            if (!current.edges.containsKey(prefix.charAt(i))) {
+                return 0;
+            }
+            current = current.edges.get(prefix.charAt(i));
+        }
+
+        return current.numOfContinuations;
+    }
+
+    /**
+     * Returns number of added strings in trie.
+     */
+    public int size() {
+        return root.numOfContinuations;
+    }
+
+    /**
+     * Removes specified string from trie.
+     *
+     * @param element a string to remove
+     * @return true if there was specified string in trie, and false otherwise
+     */
+    public boolean remove(String element) {
+        if (!contains(element)) {
+            return false;
+        }
+
+        Node current = root;
+        for (int i = 0; i < element.length(); i++) {
+            current.numOfContinuations--;
+            Node next = current.edges.get(element.charAt(i));
+            if (next.numOfContinuations == 1) {
+                current.edges.remove(element.charAt(i));
+            }
+            current = next;
+        }
+        current.numOfContinuations--;
+        current.isTerminal = false;
+
+        return true;
+    }
+
+    /**
+     * Serializes current trie to given stream.
+     *
+     * @param out stream for writing
+     */
+    public void serialize(OutputStream out) throws IOException {
+        ObjectOutputStream objectOutputStream = new ObjectOutputStream(out);
+        objectOutputStream.writeObject(this);
+    }
+
+    /**
+     * Deserializes trie from given stream and replaces current trie with new one.
+     *
+     * @param in stream for reading
+     */
+    public void deserialize(InputStream in) throws IOException, ClassNotFoundException {
+        ObjectInputStream objectInputStream = new ObjectInputStream(new BufferedInputStream(in));
+        root = ((Trie) objectInputStream.readObject()).root;
+    }
+
+    /**
+     * Vertex in trie. Stores outgoing edges with corresponding characters.
+     */
+    private class Node implements Serializable {
+        private Hashtable<Character, Node> edges = new Hashtable<Character, Node>();
+        private boolean isTerminal = false;
+        private int numOfContinuations = 0;
+    }
+}