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Huffman Coding using Priority Queue
• Difficulty Level : Medium
• Last Updated : 11 Sep, 2020

Prerequisite: Greedy Algorithms | Set 3 (Huffman Coding), priority_queue::push() and priority_queue::pop() in C++ STL
Given a char array ch[] and frequency of each characters as freq[]. The task is to find Huffman Codes for every character in ch[] using Priority Queue.

Example

Input: ch[] = { ‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’ }, freq[] = { 5, 9, 12, 13, 16, 45 }
Output:
f 0
c 100
d 101
a 1100
b 1101
e 111

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach:

1. Push all the characters in ch[] mapped to corresponding frequncy freq[] in priority queue.
2. To create Huffman Tree, pop two nodes from priority queue.
3. Assign two popped node from priority queue as left and right child of new node.
4. Push the new node formed in priority queue.
5. Repeat all above steps untill size of priority queue becomes 1.
6. Traverse the Huffman Tree (whose root is the only node left in the priority queue) to store the Huffman Code

for each character in ch[].

7. Print all the stored Huffman Code for every character in ch[].

Below is the implementation of the above approach:

 `// C++ Program for Huffman Coding``// using Priority Queue``#include ``#include ``using` `namespace` `std;`` ` `// Maximum Height of Huffman Tree.``#define MAX_SIZE 100`` ` `class` `HuffmanTreeNode {``public``:``    ``// Stores character``    ``char` `data;`` ` `    ``// Stores frequency of``    ``// the character``    ``int` `freq;`` ` `    ``// Left child of the``    ``// current node``    ``HuffmanTreeNode* left;`` ` `    ``// Right child of the``    ``// current node``    ``HuffmanTreeNode* right;`` ` `    ``// Initializing the``    ``// current node``    ``HuffmanTreeNode(``char` `character,``                    ``int` `frequency)``    ``{``        ``data = character;``        ``freq = frequency;``        ``left = right = NULL;``    ``}``};`` ` `// Custom comparator class``class` `Compare {``public``:``    ``bool` `operator()(HuffmanTreeNode* a,``                    ``HuffmanTreeNode* b)``    ``{``        ``// Defining priority on``        ``// the basis of frequency``        ``return` `a->freq > b->freq;``    ``}``};`` ` `// Function to generate Huffman``// Encoding Tree``HuffmanTreeNode* generateTree(priority_queue,``                                             ``Compare> pq)``{`` ` `    ``// We keep on looping till``    ``// only one node remains in``    ``// the Priority Queue``    ``while` `(pq.size() != 1) {`` ` `        ``// Node which has least``        ``// frequency``        ``HuffmanTreeNode* left = pq.top();`` ` `        ``// Remove node from``        ``// Priority Queue``        ``pq.pop();`` ` `        ``// Node which has least``        ``// frequency``        ``HuffmanTreeNode* right = pq.top();`` ` `        ``// Remove node from``        ``// Priority Queue``        ``pq.pop();`` ` `        ``// A new node is formed``        ``// with frequency left->freq``        ``// + right->freq`` ` `        ``// We take data as '\$'``        ``// because we are only``        ``// concerned with the``        ``// frequency``        ``HuffmanTreeNode* node = ``new` `HuffmanTreeNode(``'\$'``,``                                  ``left->freq + right->freq);``        ``node->left = left;``        ``node->right = right;`` ` `        ``// Push back node``        ``// created to the``        ``// Priority Queue``        ``pq.push(node);``    ``}`` ` `    ``return` `pq.top();``}`` ` `// Function to print the``// huffman code for each``// character.`` ` `// It uses arr to store the codes``void` `printCodes(HuffmanTreeNode* root,``                ``int` `arr[], ``int` `top)``{``    ``// Assign 0 to the left node``    ``// and recur``    ``if` `(root->left) {``        ``arr[top] = 0;``        ``printCodes(root->left,``                   ``arr, top + 1);``    ``}`` ` `    ``// Assign 1 to the right``    ``// node and recur``    ``if` `(root->right) {``        ``arr[top] = 1;``        ``printCodes(root->right, arr, top + 1);``    ``}`` ` `    ``// If this is a leaf node,``    ``// then we print root->data`` ` `    ``// We also print the code``    ``// for this character from arr``    ``if` `(!root->left && !root->right) {``        ``cout << root->data << ``" "``;``        ``for` `(``int` `i = 0; i < top; i++) {``            ``cout << arr[i];``        ``}``        ``cout << endl;``    ``}``}`` ` `void` `HuffmanCodes(``char` `data[],``                  ``int` `freq[], ``int` `size)``{`` ` `    ``// Declaring priority queue``    ``// using custom comparator``    ``priority_queue,``                   ``Compare>``        ``pq;`` ` `    ``// Populating the priority``    ``// queue``    ``for` `(``int` `i = 0; i < size; i++) {``        ``HuffmanTreeNode* newNode``            ``= ``new` `HuffmanTreeNode(data[i], freq[i]);``        ``pq.push(newNode);``    ``}`` ` `    ``// Generate Huffman Encoding``    ``// Tree and get the root node``    ``HuffmanTreeNode* root = generateTree(pq);`` ` `    ``// Print Huffman Codes``    ``int` `arr[MAX_SIZE], top = 0;``    ``printCodes(root, arr, top);``}`` ` `// Driver Code``int` `main()``{``    ``char` `data[] = { ``'a'``, ``'b'``, ``'c'``, ``'d'``, ``'e'``, ``'f'` `};``    ``int` `freq[] = { 5, 9, 12, 13, 16, 45 };``    ``int` `size = ``sizeof``(data) / ``sizeof``(data);`` ` `    ``HuffmanCodes(data, freq, size);``    ``return` `0;``}`
Output:
```f 0
c 100
d 101
a 1100
b 1101
e 111
```

Time Complexity: O(n*logn) where n is the number of unique characters

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