Merge pull request #1256 from BenceLakos/feature/ternary

Implement ternary search

Author: StepfenShawn

src/main/java/com/search/TernarySearch.java

@@ -0,0 +1,57 @@
+package com.search;
+
+/**
+ * A ternary search algorithm is a technique in computer science for finding the minimum or maximum of a unimodal function
+ * The algorithm determines either that the minimum or maximum cannot be in the first third of the domain
+ * or that it cannot be in the last third of the domain, then repeats on the remaining third.
+ * <p>
+ * Worst-case performance	Θ(log3(N))
+ * Best-case performance	O(1)
+ * Average performance	Θ(log3(N))
+ * Worst-case space complexity	O(1)
+ */
+public final class TernarySearch {
+
+    /**
+     * @param arr   The **Sorted** array in which we will search the element.
+     * @param value The value that we want to search for.
+     * @return The index of the element if found.
+     * Else returns -1.
+     */
+    public static <T extends Comparable<T>> int find(T[] arr, T value) {
+        return search(arr, value, 0, arr.length - 1);
+    }
+
+    /**
+     * @param arr   The **Sorted** array in which we will search the element.
+     * @param key   The value that we want to search for.
+     * @param start The starting index from which we will start Searching.
+     * @param end   The ending index till which we will Search.
+     * @return Returns the index of the Element if found.
+     * Else returns -1.
+     */
+    private static <T extends Comparable<T>> int search(T[] arr, T key, int start, int end) {
+        if (start > end) {
+            return -1;
+        }
+
+        int mid1 = start + (end - start) / 3;
+        int mid2 = start + 2 * (end - start) / 3;
+
+        if (key.compareTo(arr[mid1]) == 0) {
+            return mid1;
+        } else if (key.compareTo(arr[mid2]) == 0) {
+            return mid2;
+        }
+        else if (key.compareTo(arr[mid1]) < 0) {
+            return search(arr, key, start, --mid1);
+        }
+        else if (key.compareTo(arr[mid2]) > 0) {
+            return search(arr, key, ++mid2, end);
+        }
+        else {
+            return search(arr, key, mid1, mid2);
+        }
+    }
+
+}

src/test/java/com/search/TernarySearchTest.java

@@ -0,0 +1,26 @@
+package com.search;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+public class TernarySearchTest {
+
+    @Test
+    void testTernarySearch() {
+        Integer[] arr1 = {1, 2, 3, 5, 8, 13, 21, 34, 55};
+        Assertions.assertEquals(2, TernarySearch.find(arr1, 3), "Incorrect index");
+        Assertions.assertEquals(0, TernarySearch.find(arr1, 1), "Incorrect index");
+        Assertions.assertEquals(8, TernarySearch.find(arr1, 55), "Incorrect index");
+        Assertions.assertEquals(-1, TernarySearch.find(arr1, -2), "Incorrect index");
+        Assertions.assertEquals(-1, TernarySearch.find(arr1, 4), "Incorrect index");
+
+        String[] arr2 = {"A", "B", "C", "D"};
+        Assertions.assertEquals(2, TernarySearch.find(arr2, "C"), "Incorrect index");
+        Assertions.assertEquals(1, TernarySearch.find(arr2, "B"), "Incorrect index");
+        Assertions.assertEquals(-1, TernarySearch.find(arr2, "F"), "Incorrect index");
+
+        String[] arr3 = {};
+        Assertions.assertEquals(-1, TernarySearch.find(arr3, ""), "Incorrect index");
+    }
+
+}