Added Ternary Search Tree (TST) implementation with tests

Author: Sarabjeet13

src/main/java/com/thealgorithms/datastructures/trees/TernarySearchTree.java

@@ -0,0 +1,81 @@
+package main.java.com.thealgorithms.datastructures.trees;
+
+/**
+ * Ternary Search Tree (TST) implementation.
+ * Stores strings and supports insertion, search, and prefix checking.
+ *
+ * Time Complexity:
+ * - Insert: O(m), where m = word length
+ * - Search: O(m)
+ */
+public class TernarySearchTree {
+    private Node root;
+
+    private static class Node {
+        char character;
+        boolean isEndOfWord;
+        Node left, middle, right;
+
+        Node(char character) {
+            this.character = character;
+        }
+    }
+
+    /** Insert a word into the TST */
+    public void insert(String word) {
+        root = insert(root, word, 0);
+    }
+
+    private Node insert(Node node, String word, int index) {
+        char ch = word.charAt(index);
+        if (node == null) {
+            node = new Node(ch);
+        }
+
+        if (ch < node.character) {
+            node.left = insert(node.left, word, index);
+        } else if (ch > node.character) {
+            node.right = insert(node.right, word, index);
+        } else {
+            if (index + 1 < word.length()) {
+                node.middle = insert(node.middle, word, index + 1);
+            } else {
+                node.isEndOfWord = true;
+            }
+        }
+        return node;
+    }
+
+    /** Search if a word exists in TST */
+    public boolean search(String word) {
+        return search(root, word, 0);
+    }
+
+    private boolean search(Node node, String word, int index) {
+        if (node == null) return false;
+
+        char ch = word.charAt(index);
+        if (ch < node.character) {
+            return search(node.left, word, index);
+        } else if (ch > node.character) {
+            return search(node.right, word, index);
+        } else {
+            if (index + 1 == word.length()) {
+                return node.isEndOfWord;
+            }
+            return search(node.middle, word, index + 1);
+        }
+    }
+
+    // Example usage
+    public static void main(String[] args) {
+        TernarySearchTree tst = new TernarySearchTree();
+        tst.insert("cat");
+        tst.insert("cap");
+        tst.insert("bat");
+
+        System.out.println(tst.search("cat")); // true
+        System.out.println(tst.search("cap")); // true
+        System.out.println(tst.search("can")); // false
+    }
+}

src/main/java/com/thealgorithms/datastructures/trees/TernarySearchTreeTest.java

@@ -0,0 +1,61 @@
+package main.java.com.thealgorithms.datastructures.trees;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+/**
+ * Unit tests for TernarySearchTree.
+ */
+class TernarySearchTreeTest {
+
+    @Test
+    void testInsertAndSearchWords() {
+        TernarySearchTree tst = new TernarySearchTree();
+
+        tst.insert("cat");
+        tst.insert("cap");
+        tst.insert("bat");
+
+        assertTrue(tst.search("cat"));
+        assertTrue(tst.search("cap"));
+        assertTrue(tst.search("bat"));
+
+        assertFalse(tst.search("can"));
+        assertFalse(tst.search("dog"));
+    }
+
+    @Test
+    void testSingleCharacterWord() {
+        TernarySearchTree tst = new TernarySearchTree();
+
+        tst.insert("a");
+        tst.insert("b");
+
+        assertTrue(tst.search("a"));
+        assertTrue(tst.search("b"));
+        assertFalse(tst.search("c"));
+    }
+
+    @Test
+    void testPrefixNotWholeWord() {
+        TernarySearchTree tst = new TernarySearchTree();
+
+        tst.insert("hello");
+
+        assertFalse(tst.search("he"));     // prefix only
+        assertFalse(tst.search("hell"));   // prefix only
+        assertTrue(tst.search("hello"));   // complete word
+    }
+
+    @Test
+    void testMultipleInsertionsOfSameWord() {
+        TernarySearchTree tst = new TernarySearchTree();
+
+        tst.insert("java");
+        tst.insert("java");
+        tst.insert("java");
+
+        assertTrue(tst.search("java"));
+        assertFalse(tst.search("javac"));
+    }
+}