Huffman Tree (use priority queue) in C++

Use the priority queue to implement Huffman Tree, written in C++ and use STL.

#include <iostream>
#include <stdlib.h>
#include <queue>
#include <vector>
using namespace std;

struct Node
{
    int val;
    struct Node * left;
    struct Node * right;
};

typedef struct Node * p_Node;

struct cmp
{
    bool operator () (p_Node const &p1, p_Node const &p2)
    {
        return p1->val > p2->val;
    }
};

p_Node createNode(int val, p_Node left, p_Node right)
{
    p_Node node = (p_Node)malloc(sizeof(struct Node));
    node->val = val;
    node->left = left;
    node->right = right;

    return node;
}

p_Node buildTree(int * vec, int n)
{
    priority_queue<p_Node, vector<p_Node>, cmp> forest;

    for(int i = 0; i < n; i++)
    {
        p_Node node = createNode(vec[i], NULL, NULL);
        forest.push(node);
    }

    while(forest.size() > 1)
    {
        p_Node node1 =  forest.top();
        forest.pop();
        p_Node node2 =  forest.top();
        forest.pop();
        cout<<node1->val<<"  "<<node2->val<<endl;
        p_Node new_node = createNode(node1->val + node2->val, node1, node2);
        forest.push(new_node);
    }
    return forest.top();
}