Implement Heap Sort algorithm with heapify function and test cases

Author: Karanjot786

Python/algorithms/sorting/heap_sort.py

@@ -0,0 +1,120 @@
+"""
+Heap Sort Algorithm
+Time Complexity: O(n log n) in all cases
+Space Complexity: O(1) - sorts in place
+Author: Karanjot Singh
+Date: October 2025
+Hacktoberfest 2025
+"""
+
+def heapify(arr, n, i):
+    """
+    Heapify subtree rooted at index i
+
+    Args:
+        arr: Array to heapify
+        n: Size of heap
+        i: Root index of subtree
+    """
+    largest = i
+    left = 2 * i + 1
+    right = 2 * i + 2
+
+    # Check if left child exists and is greater than root
+    if left < n and arr[left] > arr[largest]:
+        largest = left
+
+    # Check if right child exists and is greater than largest so far
+    if right < n and arr[right] > arr[largest]:
+        largest = right
+
+    # If largest is not root, swap and continue heapifying
+    if largest != i:
+        arr[i], arr[largest] = arr[largest], arr[i]
+        heapify(arr, n, largest)
+
+
+def heap_sort(arr):
+    """
+    Heap sort implementation
+
+    Steps:
+    1. Build max heap from array
+    2. Extract elements one by one from heap
+    3. Place extracted element at the end
+
+    Args:
+        arr: List to sort (modified in place)
+    """
+    n = len(arr)
+
+    # Build max heap
+    # Start from last non-leaf node and heapify each node
+    for i in range(n // 2 - 1, -1, -1):
+        heapify(arr, n, i)
+
+    # Extract elements from heap one by one
+    for i in range(n - 1, 0, -1):
+        # Move current root to end
+        arr[0], arr[i] = arr[i], arr[0]
+
+        # Heapify the reduced heap
+        heapify(arr, i, 0)
+
+
+def heap_sort_copy(arr):
+    """
+    Heap sort that returns sorted copy
+
+    Args:
+        arr: List to sort
+
+    Returns:
+        Sorted copy of the list
+    """
+    arr_copy = arr.copy()
+    heap_sort(arr_copy)
+    return arr_copy
+
+
+# Test cases
+if __name__ == "__main__":
+    print("=== Heap Sort Algorithm ===\n")
+
+    # Test case 1: Random array
+    test1 = [12, 11, 13, 5, 6, 7]
+    print(f"Original array: {test1}")
+    sorted1 = heap_sort_copy(test1)
+    print(f"Sorted array:   {sorted1}")
+
+    # Test case 2: Already sorted
+    test2 = [1, 2, 3, 4, 5]
+    print(f"\nOriginal array: {test2}")
+    sorted2 = heap_sort_copy(test2)
+    print(f"Sorted array:   {sorted2}")
+
+    # Test case 3: Reverse sorted
+    test3 = [10, 8, 6, 4, 2]
+    print(f"\nOriginal array: {test3}")
+    sorted3 = heap_sort_copy(test3)
+    print(f"Sorted array:   {sorted3}")
+
+    # Test case 4: Array with duplicates
+    test4 = [4, 10, 3, 5, 1, 3, 10]
+    print(f"\nOriginal array: {test4}")
+    sorted4 = heap_sort_copy(test4)
+    print(f"Sorted array:   {sorted4}")
+
+    # Test case 5: Large array
+    test5 = [64, 34, 25, 12, 22, 11, 90, 88, 45, 50, 23, 36]
+    print(f"\nOriginal array: {test5}")
+    sorted5 = heap_sort_copy(test5)
+    print(f"Sorted array:   {sorted5}")
+
+    # Test case 6: Single element
+    test6 = [42]
+    print(f"\nOriginal array: {test6}")
+    sorted6 = heap_sort_copy(test6)
+    print(f"Sorted array:   {sorted6}")
+
+    print("\n✅ All test cases completed!")