Min heap does not work as intended in C++ - c++

Question

I am having problem with the min heap algorithm.
The code I programmed returned an array that incorrectly arranged, below is my code, can someone help me to find where should I change?
Thank you.
The data was under ELEMENT arr[], where ELEMENT is a struct:
struct ELEMENT{
int key;
};
void constructMinHeap(ELEMENT arr[], int size, int num){
int left = leftNode(num);
int right = rightNode(num);
int min;
if (left < size && arr[left].key < arr[num].key){
min = left;
} else{
min = num;
}
if (right < size && arr[right].key < arr[min].key){
min = right;
}
if(min != num){
swap(&arr[num].key, &arr[min].key);
constructMinHeap(arr, size, num);
}
}
ELEMENT* buildHelper (ELEMENT arr[], int size){
int i = size / 2 - 1;
while (i >= 0){
constructMinHeap(arr, size, i);
i--;
}
return arr;
}
And here is the code for calculating leftNode and rightNode:
int leftNode(int num){
return 2 * num + 1;
}
int rightNode(int num){
return (2 * num) + 2;
}
The following code is for storing data into the ELEMENT arr[]:
ifstream infile("somefilename.txt");
if (infile.good()) {
int i = 0;
int size;
infile >> size;
ELEMENT arr[size];
while (i < size){
infile >> arr[i].key;
i++;
}
BuildHeap(h, arr, size);
infile.close();
} else{
cout << "File \"somefilename.txt\" does not exist in the current directory.";
}
When the file's content is
4 4 3 2 1
The feedback becomes:
1 4 2 3

Answer 1

Your recursive call for num is incorrect, it has no progress since it calls the same num again. In case if the node is swapped with its smallest child, that child should be further pushed down until it takes its proper place. This will guarantee that the binary subtree rooted at num is heapified
The correct recursion is the following (you didn't show the swap so I added my own one):
void swap(ELEMENT* e1, ELEMENT* e2)
{
int tmp = e1->key;
e1->key = e2->key;
e2->key = tmp;
}
void constructMinHeap(ELEMENT arr[], int size, int num){
int left = leftNode(num);
int right = rightNode(num);
int min = num;
if (left < size && arr[left].key < arr[min].key){
min = left;
}
if (right < size && arr[right].key < arr[min].key){
min = right;
}
if(min != num)
{
swap(&arr[num], &arr[min]);
constructMinHeap(arr, size, min);
}
}