refactor: MajorityElement (#6380)

* refactor: MajorityElement

* refactor: fix import ordering

---------

Co-authored-by: Deniz Altunkapan <[email protected]>

Author: alxkm

src/main/java/com/thealgorithms/datastructures/hashmap/hashing/MajorityElement.java

@@ -1,8 +1,10 @@
 package com.thealgorithms.datastructures.hashmap.hashing;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 /**
  * This class provides a method to find the majority element(s) in an array of integers.
@@ -18,13 +20,18 @@ private MajorityElement() {
      * Returns a list of majority element(s) from the given array of integers.
      *
      * @param nums an array of integers
-     * @return a list containing the majority element(s); returns an empty list if none exist
+     * @return a list containing the majority element(s); returns an empty list if none exist or input is null/empty
      */
     public static List<Integer> majority(int[] nums) {
-        HashMap<Integer, Integer> numToCount = new HashMap<>();
+        if (nums == null || nums.length == 0) {
+            return Collections.emptyList();
+        }
+
+        Map<Integer, Integer> numToCount = new HashMap<>();
         for (final var num : nums) {
             numToCount.merge(num, 1, Integer::sum);
         }
+
         List<Integer> majorityElements = new ArrayList<>();
         for (final var entry : numToCount.entrySet()) {
             if (entry.getValue() >= nums.length / 2) {