Add Majority Element (#4131)

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

@@ -0,0 +1,34 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import java.util.HashMap;
+import java.util.List;
+import java.util.ArrayList;
+/*
+This class finds the majority element(s) in an array of integers.
+A majority element is an element that appears more than or equal to n/2 times, where n is the length of the array.
+*/
+public class MajorityElement {
+      /*
+     This method returns the majority element(s) in the given array of integers.
+     @param nums: an array of integers
+     @return a list of majority elements
+     */
+    public static List<Integer> majority(int[] nums){
+        HashMap<Integer,Integer> numToCount  = new HashMap<>();
+        int n = nums.length;
+        for (int i = 0; i < n; i++) {
+            if (numToCount.containsKey(nums[i])){
+                numToCount.put(nums[i],numToCount.get(nums[i])+1);
+            } else {
+                numToCount.put(nums[i],1);
+            }
+        }
+        List<Integer> majorityElements = new ArrayList<>();
+        for (int key: numToCount.keySet()) {
+            if (numToCount.get(key) >= n/2){
+                majorityElements.add(key);
+            }
+        }
+        return majorityElements;
+    }
+}

src/test/java/com/thealgorithms/datastructures/hashmap/hashing/MajorityElementTest.java

@@ -0,0 +1,49 @@
+package com.thealgorithms.datastructures.hashmap.hashing;
+
+import com.thealgorithms.datastructures.hashmap.hashing.MajorityElement;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.List;
+
+import java.util.ArrayList;
+
+public class MajorityElementTest{
+    @Test
+    void testMajorityWithSingleMajorityElement() {
+        int[] nums = {1, 2, 3, 9, 9, 6, 7, 8, 9, 9, 9, 9};
+        List<Integer> expected = new ArrayList<>();
+        expected.add(9);
+        List<Integer> actual = MajorityElement.majority(nums);
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    void testMajorityWithMultipleMajorityElements() {
+        int[] nums = {1, 2, 3, 3, 4, 4, 4, 4};
+        List<Integer> expected = new ArrayList<>();
+        expected.add(4);
+        List<Integer> actual = MajorityElement.majority(nums);
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    void testMajorityWithNoMajorityElement() {
+        int[] nums = {1, 2, 4, 4, 5, 4};
+        List<Integer> expected = new ArrayList<>();
+        expected.add(4);
+        List<Integer> actual = MajorityElement.majority(nums);
+        assertEquals(expected, actual);
+    }
+
+    @Test
+    void testMajorityWithEmptyArray() {
+        int[] nums = {};
+        List<Integer> expected = Collections.emptyList(); 
+        List<Integer> actual = MajorityElement.majority(nums);
+        assertEquals(expected, actual);
+    }
+}