docs(searches): document null return behavior, update best-case complexity, and test UpperBound duplicates

Author: hardikkaurani

src/main/java/com/thealgorithms/searches/LowerBound.java

@@ -13,7 +13,7 @@
  * This is an extension of BinarySearch.
  *
  * <p>
- * Worst-case performance O(log n) Best-case performance O(1) Average
+ * Worst-case performance O(log n) Best-case performance O(log n) Average
  * performance O(log n) Worst-case space complexity O(1)
  *
  * @author Pratik Padalia (https://github.com/15pratik)
@@ -26,7 +26,7 @@ class LowerBound implements SearchAlgorithm {
      * @param array is an array where the LowerBound value is to be found
      * @param key is an element for which the LowerBound is to be found
      * @param <T> is any comparable type
-     * @return index of the LowerBound element
+     * @return index of the first element not less than key, or 0 if array is null/empty or key is null
      */
     @Override
     public <T extends Comparable<T>> int find(T[] array, T key) {

src/main/java/com/thealgorithms/searches/UpperBound.java

@@ -13,7 +13,7 @@
  * This is an extension of BinarySearch.
  *
  * <p>
- * Worst-case performance O(log n) Best-case performance O(1) Average
+ * Worst-case performance O(log n) Best-case performance O(log n) Average
  * performance O(log n) Worst-case space complexity O(1)
  *
  * @author Pratik Padalia (https://github.com/15pratik)
@@ -26,7 +26,7 @@ class UpperBound implements SearchAlgorithm {
      * @param array is an array where the UpperBound value is to be found
      * @param key is an element for which the UpperBound is to be found
      * @param <T> is any comparable type
-     * @return index of the UpperBound element
+     * @return index of the first element greater than key, last index if no such element exists, or 0 if array is null/empty or key is null
      */
     @Override
     public <T extends Comparable<T>> int find(T[] array, T key) {

src/test/java/com/thealgorithms/searches/UpperBoundTest.java

@@ -57,6 +57,15 @@ void testUpperBoundMultipleValues() {
         assertTrue(arrayWithDuplicates[index] > key, "Element at the upper bound index should be greater than the key.");
     }
 
+    @Test
+    void testUpperBoundDuplicates() {
+        Integer[] arrayWithDuplicates = {1, 2, 2, 2, 3};
+        assertEquals(4, upperBound.find(arrayWithDuplicates, 2), "Upper bound for 2 in {1, 2, 2, 2, 3} should be at index 4");
+
+        Integer[] allSame = {2, 2, 2, 2, 2};
+        assertEquals(4, upperBound.find(allSame, 2), "Upper bound for 2 in all-same array should be at last index 4");
+    }
+
     @Test
     void testUpperBoundLowerThanMin() {
         int key = 0; // Test with a key lower than the minimum element