Priority Queue Wrong Order - c++

Question

I am programming the huffman encoding. This is the beginning of my program:
using namespace std;
//Counting methods
int *CountCharOccurence(string text)
{
int *charOccurrence = new int[127];
for(int i = 0; i < text.length(); i++)
{
charOccurrence[text[i]]++;
}
return charOccurrence;
}
void DisplayCharOccurence(int *charOccurrence)
{
for(int i = 0; i < 127; i++)
{
if(charOccurrence[i] > 0)
{
cout << (char)i << ": " << charOccurrence[i] << endl;
}
}
}
//Node struct
struct Node
{
public:
char character;
int occurrence;
Node(char c, int occ) {
character = c;
occurrence = occ;
}
bool operator < (const Node* node)
{
return (occurrence < node->occurrence);
}
};
void CreateHuffmanTree(int *charOccurrence)
{
priority_queue<Node*, vector<Node*> > pq;
for(int i = 0; i < 127; i++)
{
if(charOccurrence[i])
{
Node* node = new Node((char)i, charOccurrence[i]);
pq.push(node);
}
}
//Test
while(!pq.empty())
{
cout << "peek: " << pq.top()->character << pq.top()->occurrence << endl;
pq.pop();
}
}
int main(int argc, char** argv) {
int *occurrenceArray;
occurrenceArray = CountCharOccurence("SUSIE SAYS IT IS EASY");
DisplayCharOccurence(occurrenceArray);
CreateHuffmanTree(occurrenceArray);
return (EXIT_SUCCESS);
}
The program first outputs the characters with their occurence number. This looks fine:
: 4
A: 2
E: 2
I: 3
S: 6
T: 1
U: 1
Y: 2
but the test loop which has to display the node contents in priority order outputs this:
peek: Y2
peek: U1
peek: S6
peek: T1
peek: I3
peek: E2
peek: 4
peek: A2
This is not the expected order. Why?

Answer 1

Elements in your priority queue are pointers. Since you don't provide a function that takes 2 pointers to Node objects , default compare function compares 2 pointers.
bool compareNodes(Node* val1, Node* val2)
{
return val1->occurence < val2->occurence;
}
priority_queue<Node*, vector<Node*>,compareNodes > pq;
Your operator < is used when Node compares with Node*

Answer 2

You should tell your priority queue what it should sort by. In your case, you have to tell it to sort by Node::occurence.

Answer 3

You are storing pointers to nodes in the queue, but haven't provided a suitable comparison function, so they are sorted by comparing the pointers. The operator< you've provided will compare a node with a pointer, which isn't what you want.
There are two options:
Provide a function for comparing two node pointers according to their values, and give this function to the queue, or
Store node objects in the queue, and provide an operator< to compare two nodes.
The second option will also fix the memory leak in your code, and remove a whole pile of unnecessary memory allocations, so I would suggest that.