From 4f464429fcfce90776bbd3807265b12ec0f4a107 Mon Sep 17 00:00:00 2001
From: Aditi Chaurasia <103958459+aditi-chaurasia@users.noreply.github.com>
Date: Thu, 20 Oct 2022 00:45:19 +0530
Subject: [PATCH] Create SortByFreq.cpp

Hacktoberfest Submission. #13
---
 Cpp/SortByFreq.cpp | 119 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 119 insertions(+)
 create mode 100644 Cpp/SortByFreq.cpp

diff --git a/Cpp/SortByFreq.cpp b/Cpp/SortByFreq.cpp
new file mode 100644
index 0000000..9c5d8c4
--- /dev/null
+++ b/Cpp/SortByFreq.cpp
@@ -0,0 +1,119 @@
+// Implementation of above algorithm in C++.
+#include <iostream>
+#include <stdlib.h>
+using namespace std;
+
+/* A BST node has data, freq, left and right pointers */
+struct BSTNode
+{
+	struct BSTNode *left;
+	int data;
+	int freq;
+	struct BSTNode *right;
+};
+
+// A structure to store data and its frequency
+struct dataFreq
+{
+	int data;
+	int freq;
+};
+
+/* Function for qsort() implementation. Compare frequencies to
+sort the array according to decreasing order of frequency */
+int compare(const void *a, const void *b)
+{
+	return ( (*(const dataFreq*)b).freq - (*(const dataFreq*)a).freq );
+}
+
+/* Helper function that allocates a new node with the given data,
+frequency as 1 and NULL left and right pointers.*/
+BSTNode* newNode(int data)
+{
+	struct BSTNode* node = new BSTNode;
+	node->data = data;
+	node->left = NULL;
+	node->right = NULL;
+	node->freq = 1;
+	return (node);
+}
+
+// A utility function to insert a given key to BST. If element
+// is already present, then increases frequency
+BSTNode *insert(BSTNode *root, int data)
+{
+	if (root == NULL)
+		return newNode(data);
+	if (data == root->data) // If already present
+		root->freq += 1;
+	else if (data < root->data)
+		root->left = insert(root->left, data);
+	else
+		root->right = insert(root->right, data);
+	return root;
+}
+
+// Function to copy elements and their frequencies to count[].
+void store(BSTNode *root, dataFreq count[], int *index)
+{
+	// Base Case
+	if (root == NULL) return;
+
+	// Recur for left subtree
+	store(root->left, count, index);
+
+	// Store item from root and increment index
+	count[(*index)].freq = root->freq;
+	count[(*index)].data = root->data;
+	(*index)++;
+
+	// Recur for right subtree
+	store(root->right, count, index);
+}
+
+// The main function that takes an input array as an argument
+// and sorts the array items according to frequency
+void sortByFrequency(int arr[], int n)
+{
+	// Create an empty BST and insert all array items in BST
+	struct BSTNode *root = NULL;
+	for (int i = 0; i < n; ++i)
+		root = insert(root, arr[i]);
+
+	// Create an auxiliary array 'count[]' to store data and
+	// frequency pairs. The maximum size of this array would
+	// be n when all elements are different
+	dataFreq count[n];
+	int index = 0;
+	store(root, count, &index);
+
+	// Sort the count[] array according to frequency (or count)
+	qsort(count, index, sizeof(count[0]), compare);
+
+	// Finally, traverse the sorted count[] array and copy the
+	// i'th item 'freq' times to original array 'arr[]'
+	int j = 0;
+	for (int i = 0; i < index; i++)
+	{
+		for (int freq = count[i].freq; freq > 0; freq--)
+			arr[j++] = count[i].data;
+	}
+}
+
+// A utility function to print an array of size n
+void printArray(int arr[], int n)
+{
+	for (int i = 0; i < n; i++)
+		cout << arr[i] << " ";
+	cout << endl;
+}
+
+/* Driver program to test above functions */
+int main()
+{
+	int arr[] = {2, 3, 2, 4, 5, 12, 2, 3, 3, 3, 12};
+	int n = sizeof(arr)/sizeof(arr[0]);
+	sortByFrequency(arr, n);
+	printArray(arr, n);
+	return 0;
+}