Add another method to find kth number (#5918)

Author: TaranjeetSinghKalsi

src/main/java/com/thealgorithms/maths/FindKthNumber.java

@@ -1,5 +1,7 @@
 package com.thealgorithms.maths;
 
+import java.util.Collections;
+import java.util.PriorityQueue;
 import java.util.Random;
 
 /**
@@ -62,4 +64,19 @@ private static void swap(int[] array, int i, int j) {
         array[i] = array[j];
         array[j] = temp;
     }
+
+    public static int findKthMaxUsingHeap(int[] array, int k) {
+        if (k <= 0 || k > array.length) {
+            throw new IllegalArgumentException("k must be between 1 and the size of the array");
+        }
+        PriorityQueue<Integer> maxHeap = new PriorityQueue<>(Collections.reverseOrder()); // using max-heap to store numbers.
+        for (int num : array) {
+            maxHeap.add(num);
+        }
+        while (k > 1) {
+            maxHeap.poll(); // removing max number from heap
+            k--;
+        }
+        return maxHeap.peek();
+    }
 }

src/test/java/com/thealgorithms/maths/FindKthNumberTest.java

@@ -14,37 +14,50 @@ public void testFindKthMaxTypicalCases() {
         assertEquals(3, FindKthNumber.findKthMax(array1, 3));
         assertEquals(4, FindKthNumber.findKthMax(array1, 2));
         assertEquals(5, FindKthNumber.findKthMax(array1, 1));
+        assertEquals(3, FindKthNumber.findKthMaxUsingHeap(array1, 3));
+        assertEquals(4, FindKthNumber.findKthMaxUsingHeap(array1, 2));
+        assertEquals(5, FindKthNumber.findKthMaxUsingHeap(array1, 1));
 
         int[] array2 = {7, 5, 8, 2, 1, 6};
         assertEquals(5, FindKthNumber.findKthMax(array2, 4));
         assertEquals(6, FindKthNumber.findKthMax(array2, 3));
         assertEquals(8, FindKthNumber.findKthMax(array2, 1));
+        assertEquals(5, FindKthNumber.findKthMaxUsingHeap(array2, 4));
+        assertEquals(6, FindKthNumber.findKthMaxUsingHeap(array2, 3));
+        assertEquals(8, FindKthNumber.findKthMaxUsingHeap(array2, 1));
     }
 
     @Test
     public void testFindKthMaxEdgeCases() {
         int[] array1 = {1};
         assertEquals(1, FindKthNumber.findKthMax(array1, 1));
+        assertEquals(1, FindKthNumber.findKthMaxUsingHeap(array1, 1));
 
         int[] array2 = {5, 3};
         assertEquals(5, FindKthNumber.findKthMax(array2, 1));
         assertEquals(3, FindKthNumber.findKthMax(array2, 2));
+        assertEquals(5, FindKthNumber.findKthMaxUsingHeap(array2, 1));
+        assertEquals(3, FindKthNumber.findKthMaxUsingHeap(array2, 2));
     }
 
     @Test
     public void testFindKthMaxInvalidK() {
         int[] array = {1, 2, 3, 4, 5};
         assertThrows(IllegalArgumentException.class, () -> FindKthNumber.findKthMax(array, 0));
         assertThrows(IllegalArgumentException.class, () -> FindKthNumber.findKthMax(array, 6));
+        assertThrows(IllegalArgumentException.class, () -> FindKthNumber.findKthMaxUsingHeap(array, 0));
+        assertThrows(IllegalArgumentException.class, () -> FindKthNumber.findKthMaxUsingHeap(array, 6));
     }
 
     @Test
     public void testFindKthMaxLargeArray() {
         int[] array = generateArray(1000);
         int k = new Random().nextInt(1, array.length);
         int result = FindKthNumber.findKthMax(array, k);
+        int maxK = FindKthNumber.findKthMaxUsingHeap(array, k);
         Arrays.sort(array);
         assertEquals(array[array.length - k], result);
+        assertEquals(array[array.length - k], maxK);
     }
 
     public static int[] generateArray(int capacity) {