Added documentation (TheAlgorithms#802)

* Added documentation

* Added breif comments on functions

* modified comment block

* further improved comment blocks

Author: ayaankhan98

sorting/Quick Sort.cpp

@@ -1,16 +1,42 @@
-/*
+/**
  *
  *      copyright The Algorithms
  *      Author - 
  *      Correction - ayaankhan98
  *
+ * Implementation Details -
+ *      Quick Sort is a divide and conquer algorithm. It picks and element as
+ *      pivot and partition the given array around the picked pivot. There
+ *      are many different versions of quickSort that pick pivot in different
+ *      ways.
+ *
+ *      1. Always pick the first element as pivot
+ *      2. Always pick the last element as pivot (implemented below)
+ *      3. Pick a random element as pivot
+ *      4. Pick median as pivot
+ *
+ *      The key process in quickSort is partition(). Target of partition is,
+ *      given an array and an element x(say) of array as pivot, put x at it's
+ *      correct position in sorted array and put all smaller elements (samller
+ *      than x) before x, and put all greater elements (greater than x) after
+ *      x. All this should be done in linear time
+ *
  */
 
 #include <cstdlib>
 #include <iostream>
 
+/**
+ *      This function takes last element as pivot, places  
+ *      the pivot element at its correct position in sorted  
+ *      array, and places all smaller (smaller than pivot)  
+ *      to left of pivot and all greater elements to right  
+ *      of pivot 
+ *
+ */
+
 int partition(int arr[], int low, int high) {
-    int pivot = arr[high];    // pivot
+    int pivot = arr[high];    // taking the last element as pivot
     int i = (low - 1);        // Index of smaller element
 
     for (int j = low; j < high; j++) {
@@ -29,6 +55,12 @@ int partition(int arr[], int low, int high) {
     return (i + 1);
 }
 
+/** 
+ *      The main function that implements QuickSort  
+ *      arr[] --> Array to be sorted,  
+ *      low --> Starting index,  
+ *      high --> Ending index 
+*/
 void quickSort(int arr[], int low, int high) {
     if (low < high) {
         int p = partition(arr, low, high);
@@ -37,14 +69,14 @@ void quickSort(int arr[], int low, int high) {
     }
 }
 
+// prints the array after sorting
 void show(int arr[], int size) {
     for (int i = 0; i < size; i++)
         std::cout << arr[i] << " ";
     std::cout << "\n";
 }
 
 // Driver program to test above functions
-
 int main() {
     int size;
     std::cout << "\nEnter the number of elements : ";