fix: Fix bug in pruning method

Author: 4ndrelim

docs/assets/images/Trie.png

@@ -1,7 +1,9 @@
 package dataStructures.trie;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 /**
  * Implementation of a Trie; Here we consider strings (not case-sensitive)
@@ -13,6 +15,18 @@ public Trie() {
         root = new TrieNode();
     }
 
+    private class TrieNode {
+        // CHECKSTYLE:OFF: VisibilityModifier
+        public Map<Character, TrieNode> children; // or array of size 26 (assume not case-sensitive) to denote each char
+        // CHECKSTYLE:OFF: VisibilityModifier
+        public boolean isEnd; // a marker to indicate whether the path from the root to this node forms a known word
+
+        public TrieNode() {
+            children = new HashMap<Character, TrieNode>();
+            isEnd = false;
+        }
+    }
+
     /**
      * Inserts a word into the trie.
      * @param word
@@ -90,7 +104,7 @@ public void deleteAndPrune(String word) {
             char curr = word.charAt(i);
             TrieNode nodeBeforeCurr = trackNodes.get(i);
             TrieNode nextNode = nodeBeforeCurr.children.get(curr);
-            if (nextNode.children.size() == 0) { // this node essentially doesn't track anything, remove
+            if (!nextNode.isEnd && nextNode.children.size() == 0) { // node essentially doesn't track anything, remove
                 nodeBeforeCurr.children.remove(curr);
             } else { // children.size() > 0; i.e. this node is still useful; no need to further prune upwards
                 break;

src/main/java/dataStructures/trie/README.md

@@ -7,14 +7,6 @@
 import org.junit.Assert;
 import org.junit.Test;
 
-/**
- * insert
- * search
- * delete
- * deleteAndPrune
- * wordsWithPrefix
- * getAllWords
- */
 public class TrieTest {
     @Test
     public void search_shouldFindWordIfExists() {
@@ -72,6 +64,20 @@ public void deleteAndPrune_shouldPrune() {
         Assert.assertTrue(trie.checkNodeExistsAtPosition("cs3230", 2));
     }
 
+    @Test
+    public void deleteAndPrune_shouldNotDeletePrefixWords() {
+        Trie trie = new Trie();
+        trie.insert("art");
+        trie.insert("artist");
+        trie.insert("artistic");
+        trie.insert("artistically");
+
+        Assert.assertTrue(trie.search("artistically"));
+        trie.deleteAndPrune("artistically");
+        Assert.assertTrue(trie.search("artistic")); // should not be pruned away
+        Assert.assertTrue(trie.search("art")); // should not be pruned away
+    }
+
     @Test
     public void wordsWithPrefix_shouldDisplayCurrentWordsThatMatches() {
         Trie trie = new Trie();