Quick sort implementation

quick_sort.c

#include<stdlib.h>
#include<string.h>

/**
 * Output: lines contains the partitioned array
 * @return the index of the pivot element
 */
int partition(const char **lines, int start, int end)
{
  const char *temp;
  int index;
  int store_index = start;

  // There is at least one element, use the last element as the pivot
  // lines[end]: pivot element.
  // Loop invariant:
  //   for all i : start       <= i < store_index ==> lines[i] <  lines[end]
  //   for all i : store_index <= i < index       ==> lines[i] >= lines[end]
  for (index = start; index < end; index++)
  {
    // If current element is lower than pivot
    // then swap it with the element at store_index
    // and move the store_index to the right.
    if (strcmp(lines[index], lines[end]) < 0)
    {
      temp               = lines[store_index];
      lines[store_index] = lines[index];
      lines[index]       = temp;

      store_index++;
    }
  }

  // Now move the pivot to the right place.
  temp               = lines[end];
  lines[end]         = lines[store_index];
  lines[store_index] = temp;

  return store_index;
}

void quick_sort(const char **lines, int start, int end)
{
  // If the array to sort contains at most one element, it is
  // already sorted.
  if (start >= end)
  {
    return;
  }
  // We have more than one element: partition and sort recursively.
  else
  {
    const int pivot_index = partition(lines, start, end);

    quick_sort(lines, start,           pivot_index - 1);
    quick_sort(lines, pivot_index + 1, end);
  }
}