added quick_sort_3

sorting/quick_sort_3.cpp

@@ -0,0 +1,84 @@
+/*
+Author: immortal-j
+Implementation Details
+Quick sort 3 works on Dutch National Flag Algorithm
+The major diffrence between simple quicksort and quick sort 3 comes in the
+function partition3 In quick_sort_partition3 we divide the vector/array into 3
+parts. quick sort 3 works faster in some cases as compared to simple quicksort.
+*/
+#include <iostream>
+// function to swap elements
+void swap(int *a, int *b) {
+  int temp = *a;
+  *a = *b;
+  *b = temp;
+}
+
+// function to print an array
+void print_arr(int arr[], int n) {
+  for (int i = 0; i < n; ++i)
+    std::cout << arr[i] << " ";
+  std::cout << "\n";
+}
+
+/* This function partitions arr[] in three parts
+a) arr[l..i] contains all elements smaller than pivot
+b) arr[i+1..j-1] contains all occurrences of pivot
+c) arr[j..r] contains all elements greater than pivot */
+void partition3(int arr[], int low, int high, int *i, int *j) {
+  // To handle 2 elements
+  if (high - low <= 1) {
+    if (arr[high] < arr[low])
+      swap(&arr[high], &arr[low]);
+    *i = low;
+    *j = high;
+    return;
+  }
+
+  int mid = low;
+  int pivot = arr[high];
+  while (mid <= high) {
+    if (arr[mid] < pivot)
+      swap(&arr[low++], &arr[mid++]);
+    else if (arr[mid] == pivot)
+      mid++;
+    else if (arr[mid] > pivot)
+      swap(&arr[mid], &arr[high--]);
+  }
+
+  // update i and j
+  *i = low - 1;
+  *j = mid; // or high-1
+}
+
+// 3-way partition based quick sort
+void quicksort(int arr[], int low, int high) {
+  if (low >= high) // 1 or 0 elements
+    return;
+
+  int i, j;
+
+  // i and j are passed as reference
+  partition3(arr, low, high, &i, &j);
+
+  // Recur two halves
+  quicksort(arr, low, i);
+  quicksort(arr, j, high);
+}
+
+// Driver program for above functions
+int main() {
+  int size, i;
+  std::cout << "Enter Number of elements\n";
+  std::cin >> size;
+  int *arr = new int[size];
+  for (i = 0; i < size; i++) {
+    std::cin >> arr[i];
+  }
+  quicksort(arr, 0, size - 1);
+  std::cout << "Sorted Array is:";
+  print_arr(arr, size);
+  std::cout << "\n";
+  return 0;
+
+}