Add example for quick sort

cpp/algorithms/sorting/quick/lib-quick.cpp

@@ -0,0 +1,63 @@
+/*#############################################################################
+## Author: Shaun Reed                                                        ##
+## Legal: All Content (c) 2021 Shaun Reed, all rights reserved               ##
+## About: An example implementation of quick sort using a custom library     ##
+##                                                                           ##
+## Contact: shaunrd0@gmail.com  | URL: www.shaunreed.com | GitHub: shaunrd0  ##
+###############################################################################
+*/
+
+#include "lib-quick.hpp"
+
+#include <algorithm>
+#include <vector>
+
+
+void QuickSort(std::vector<int> &array, size_t begin, size_t end)
+{
+  // Base case breaks out of recursion once begin and end converge on each other
+  if (begin >= end) return;
+
+  // Search through array, finding the correct position for array[end]
+  // + Returns the index *after* the new position for array[end]
+  size_t pivot = Partition(array, begin, end);
+  // array[end] is now at array[pivot]
+
+  // Continue to sort the lhs partition of the set
+  // + Using pivot-1 as the end index, we only consider the unsorted lhs portion
+  QuickSort(array, begin, (pivot > 0) ? pivot - 1 : pivot);
+  // In the top-level call to QuickSort, we don't reach this point until
+  // + all values in the lhs portion of the array are < array[pivot]
+
+  // At this point, pivot+1 now effectively represents the begin of unsorted rhs
+  // + From pivot+1 to end, the array is currently unsorted
+
+  // Continue to sort the rhs partition of the array
+  // + Using pivot+1 as the begin index, we only consider the unsorted rhs portion
+  QuickSort(array, pivot + 1, end);
+
+  // The entire array, or the entire partition, is now sorted
+  // + Return to previous QuickSort on call stack, or return to main
+}
+
+size_t Partition(std::vector<int> &array, size_t begin, size_t end)
+{
+  // Find the correct position for keyValue within the array partition
+  // + Partition is within the range of array[begin ... end]
+  int keyValue = array[end];
+
+  // As we find values smaller than keyValue, track the last used index
+  // + Return this value when done, so we know where the lhs partition ends
+  ssize_t lhsIndex = begin - 1;
+  // For each value within this partition, check for values < keyValue
+  for (int j = begin; j <= end - 1; j++) {
+    if (array[j] <= keyValue) {
+      // Swap all values < keyValue into the lhs portion of array
+      std::swap(array[++lhsIndex], array[j]);
+    }
+  }
+  // Swap keyValue into lhsIndex+1, since we know all values before are smaller
+  std::swap(array[++lhsIndex], array[end]);
+  // Return the last used index for the lhs partition
+  return lhsIndex;
+}