Enhance QuickSort documentation and structure

Updated QuickSort implementation with detailed comments and improved clarity.

Author: prasanth-30011

src/main/java/com/thealgorithms/sorts/QuickSort.java

@@ -1,77 +1,186 @@
 package com.thealgorithms.sorts;
 
 /**
+ * QuickSort Algorithm Implementation
+ * 
+ * QuickSort is a highly efficient sorting algorithm that uses the divide-and-conquer approach
+ * to sort elements. It was developed by C. A. R. Hoare and is commonly used in practice because
+ * of its efficiency and in-place sorting capability.
+ * 
+ * When to use QuickSort:
+ * - When you need in-place sorting (O(log n) extra space on average)
+ * - When average-case performance is important (O(n log n))
+ * - When you need to sort large datasets efficiently
+ * - When stability is not required
+ * 
+ * When NOT to use QuickSort:
+ * - When stability is required (use MergeSort instead)
+ * - When worst-case O(n^2) performance is unacceptable (use HeapSort or MergeSort)
+ * - When working with nearly sorted data (consider insertion sort or adaptive sorts)
+ * 
+ * Complexity Analysis:
+ * - Best Case: O(n log n) - when pivot divides array into two nearly equal parts
+ * - Average Case: O(n log n) - expected behavior with random pivot selection
+ * - Worst Case: O(n^2) - when pivot is always smallest/largest element (rare with random selection)
+ * - Space Complexity: O(log n) - for recursion stack in average case, O(n) in worst case
+ * 
+ * Characteristics:
+ * - In-place sorting algorithm (minimal extra space required)
+ * - Not stable (relative order of equal elements may change)
+ * - Uses randomized pivot selection to avoid worst-case scenarios
+ * - Cache-friendly due to good memory access patterns
+ * 
  * @author Varun Upadhyay (https://github.com/varunu28)
  * @author Podshivalov Nikita (https://github.com/nikitap492)
  * @see SortAlgorithm
  */
 class QuickSort implements SortAlgorithm {
 
     /**
-     * This method implements the Generic Quick Sort
+     * Main entry point for Quick Sort algorithm
+     * This method implements the generic Quick Sort for arrays of comparable objects.
+     * The algorithm works by selecting a pivot element and partitioning the array
+     * into elements smaller and larger than the pivot, then recursively sorting each partition.
      *
-     * @param array The array to be sorted Sorts the array in increasing order
+     * @param array The array to be sorted. Sorts the array in increasing order.
+     *              The array is modified in-place.
+     * @return The sorted array (same reference as input, modified in-place)
+     * @param <T> Generic type that must extend Comparable to allow element comparison
      */
     @Override
     public <T extends Comparable<T>> T[] sort(T[] array) {
+        // Start the recursive sorting process from index 0 to array.length - 1
         doSort(array, 0, array.length - 1);
         return array;
     }
 
     /**
-     * The sorting process
+     * Recursive helper method that performs the actual QuickSort sorting process.
+     * This method divides the array into smaller subarrays and recursively sorts them.
+     * The division is done using the partition method.
      *
-     * @param left The first index of an array
-     * @param right The last index of an array
-     * @param array The array to be sorted
+     * How it works:
+     * 1. If left >= right, the subarray has 0 or 1 element, so it's already sorted
+     * 2. Use randomPartition to find a pivot position that divides the array
+     * 3. Recursively sort the left subarray (elements < pivot)
+     * 4. Recursively sort the right subarray (elements >= pivot)
+     *
+     * @param array The array being sorted
+     * @param left The leftmost index of the current subarray
+     * @param right The rightmost index of the current subarray
+     * @param <T> Generic type that must extend Comparable
      */
     private static <T extends Comparable<T>> void doSort(T[] array, final int left, final int right) {
+        // Base case: if left >= right, the subarray has 0 or 1 element (already sorted)
         if (left < right) {
+            // Partition the array and get the final position of the pivot
             final int pivot = randomPartition(array, left, right);
+            
+            // Recursively sort the left partition (elements smaller than pivot)
+            // Sort from left to pivot - 1 since element at pivot is now in final position
             doSort(array, left, pivot - 1);
+            
+            // Recursively sort the right partition (elements greater than or equal to pivot)
+            // Sort from pivot to right since element at pivot is in final position
             doSort(array, pivot, right);
         }
     }
 
     /**
-     * Randomize the array to avoid the basically ordered sequences
+     * Selects a random pivot position and partitions the array around it.
+     * Using a random pivot helps avoid the O(n^2) worst-case scenario that occurs
+     * when the pivot is always the smallest or largest element (common with sorted data).
+     *
+     * The randomization strategy:
+     * - Generates a random index between left and right (inclusive)
+     * - Swaps the element at random index with the element at right position
+     * - Then performs standard partition with the element at right as pivot
+     * This ensures we avoid worst-case scenarios on already sorted or reverse sorted arrays
      *
-     * @param array The array to be sorted
-     * @param left The first index of an array
-     * @param right The last index of an array
-     * @return the partition index of the array
+     * @param array The array being partitioned
+     * @param left The leftmost index of the current subarray
+     * @param right The rightmost index of the current subarray
+     * @return The final partition index where the pivot element is now positioned
+     * @param <T> Generic type that must extend Comparable
      */
     private static <T extends Comparable<T>> int randomPartition(T[] array, final int left, final int right) {
+        // Generate a random index between left and right (inclusive)
+        // The formula: left + random_value ensures the index falls within [left, right]
         final int randomIndex = left + (int) (Math.random() * (right - left + 1));
+        
+        // Swap the element at randomIndex with the element at right position
+        // This moves our randomly selected pivot to the right for standard partition
         SortUtils.swap(array, randomIndex, right);
+        
+        // Perform standard partition with the element at right as the pivot
         return partition(array, left, right);
     }
 
     /**
-     * This method finds the partition index for an array
+     * Partitions the array around a pivot element.
+     * This is the core logic of QuickSort that divides the array into two regions:
+     * - Elements smaller than or equal to pivot (on the left)
+     * - Elements greater than pivot (on the right)
+     *
+     * Partitioning Strategy (Two-pointer approach):
+     * 1. Choose the middle element as pivot to improve performance on sorted arrays
+     * 2. Initialize left pointer at the start and right pointer at the end
+     * 3. Move left pointer right until finding an element >= pivot
+     * 4. Move right pointer left until finding an element < pivot
+     * 5. If pointers haven't crossed, swap the elements (move them to correct sides)
+     * 6. Continue until pointers meet or cross
+     * 7. Return the position where the pivot is finally placed
+     *
+     * Time Complexity of this method: O(n) where n is the size of the subarray
+     * 
+     * Example:
+     * Array: [3, 7, 2, 9, 1, 5] with pivot = 5 (middle element at index 2)
+     * After partition: [3, 2, 1, 5, 9, 7] where 5 is at its final sorted position
      *
-     * @param array The array to be sorted
-     * @param left The first index of an array
-     * @param right The last index of an array Finds the partition index of an
-     * array
+     * @param array The array being partitioned
+     * @param left The starting index of the current partition
+     * @param right The ending index of the current partition
+     * @return The index where the pivot element ends up after partitioning
+     * @param <T> Generic type that must extend Comparable
      */
     private static <T extends Comparable<T>> int partition(T[] array, int left, int right) {
+        // Select the middle element as the pivot
+        // Using middle element improves performance on partially sorted data
+        // The >>> operator is unsigned right shift, equivalent to (left + right) / 2 but avoids overflow
         final int mid = (left + right) >>> 1;
         final T pivot = array[mid];
-
+        
+        // Two-pointer partitioning approach
+        // Loop continues while left and right pointers haven't crossed each other
         while (left <= right) {
+            // Move left pointer right until we find an element >= pivot
+            // All elements to the left of final left position will be < pivot
             while (SortUtils.less(array[left], pivot)) {
                 ++left;
             }
+            
+            // Move right pointer left until we find an element < pivot
+            // All elements to the right of final right position will be >= pivot
             while (SortUtils.less(pivot, array[right])) {
                 --right;
             }
+            
+            // If pointers haven't crossed, swap the elements
+            // This places the smaller element on the left side and larger on the right side
             if (left <= right) {
+                // Perform the swap to move elements to their correct sides relative to pivot
                 SortUtils.swap(array, left, right);
-                ++left;
-                --right;
+                
+                // Move pointers to continue partitioning
+                ++left;  // Move left pointer right
+                --right; // Move right pointer left
             }
         }
+        
+        // Return the final position of the left pointer
+        // This is where we'll place the pivot in the next recursive step
+        // Elements [original_left...left-1] are < pivot
+        // Elements [left...original_right] are >= pivot
         return left;
     }
 }