I found this code from geeks for geeks for huffman coding, but I need help understanding it. First of all what does the struc compare do? It returns a bool, why cant we put the bool in a function but in a struc? It does not look like a traditional struc where you define values. Second, priority_queue, compare> minHeap; why do you have three elements in the definition of the queue? Usually when I right, a queue I just define the type of the queue, why does this queue have three types? Top( ) gets the minimum frequency in the code below, if you do not even sort the minheap how would you get the minimum frequencies? Or is it already sorted, but where? Below is the URL with the code I have questions on.

https://www.geeksforgeeks.org/greedy-algorithms-set-3-huffman-coding/ (C++ using STL) below is the copy paste code from this url

// C++ program for Huffman Coding

#include

using namespace std;

// A Huffman tree node

struct MinHeapNode {

// One of the input characters

char data;

// Frequency of the character

unsigned freq;

// Left and right child

MinHeapNode *left, *right;

MinHeapNode(char data, unsigned freq)

{

left = right = NULL;

this->data = data;

this->freq = freq;

}

};

// For comparison of

// two heap nodes (needed in min heap)

struct compare {

bool operator()(MinHeapNode* l, MinHeapNode* r)

{

return (l->freq > r->freq);

}

};

// Prints huffman codes from

// the root of Huffman Tree.

void printCodes(struct MinHeapNode* root, string str)

{

if (!root)

return;

if (root->data != '$')

cout << root->data << ": " << str << " ";

printCodes(root->left, str + "0");

printCodes(root->right, str + "1");

}

// The main function that builds a Huffman Tree and

// print codes by traversing the built Huffman Tree

void HuffmanCodes(char data[], int freq[], int size)

{

struct MinHeapNode *left, *right, *top;

// Create a min heap & inserts all characters of data[]

priority_queue, compare> minHeap;

for (int i = 0; i < size; ++i)

minHeap.push(new MinHeapNode(data[i], freq[i]));

// Iterate while size of heap doesn't become 1

while (minHeap.size() != 1) {

// Extract the two minimum

// freq items from min heap

left = minHeap.top();

minHeap.pop();

right = minHeap.top();

minHeap.pop();

// Create a new internal node with

// frequency equal to the sum of the

// two nodes frequencies. Make the

// two extracted node as left and right children

// of this new node. Add this node

// to the min heap '$' is a special value

// for internal nodes, not used

top = new MinHeapNode('$', left->freq + right->freq);

top->left = left;

top->right = right;

minHeap.push(top);

}

// Print Huffman codes using

// the Huffman tree built above

printCodes(minHeap.top(), "");

}

// Driver program to test above functions

int main()

{

char arr[] = { 'a', 'b', 'c', 'd', 'e', 'f' };

int freq[] = { 5, 9, 12, 13, 16, 45 };

int size = sizeof(arr) / sizeof(arr[0]);

HuffmanCodes(arr, freq, size);

return 0;

}