sắp síp heapsort

#include <iostream>
#include <cmath>
#define MAX 1000
using namespace std;

nhapMang(int n, int a[])
{
    for(int i = 1; i <= n; i++)
    {
        cout << "Nhap phan tu thu a[" << i << "]: ";
        cin >> a[i];
    }
}

xuatMang(int n, int a[])
{
    for(int i = 1; i <= n; i++)
    {
        cout << "Phan tu thu a[" << i << "] la: " << a[i] << endl;
    }
}

void Upheap(int a[], int position)
{
    int value = a[position];
    while (position / 2 > 0)
    {
        int parent = position / 2;
        if (a[parent] <= value)
        {
        break;
        }
        a[position] = a[parent];
        position = parent;
    }
    a[position] = value;
}

void Downheap(int a[], int heapSize, int position)
{
    int value = a[position];
    while (position * 2 <= heapSize)
    {
        int child = position * 2;
        if (child < heapSize && a[child+1] < a[child])
        {
        ++child;
        }
        if (value <= a[child])
        {
        break;
        }
        a[position] = a[child];
        position = child;
    }
    a[position] = value;
}

int main()
{
    int n, a[MAX], heap[MAX];
    cout << "Nhap so luong phan tu: ";
    cin >> n;
    cout << "\n\n\t\tNHAP MANG" << endl;
    nhapMang(n, a);

    cout << "\n\n\t\tMANG DA SAP XEP\n";
    int heapSize;
    for (int i=1; i<=n; ++i)
    {
        Upheap(a, i);
    }
    heapSize = n;
    for (int i=n; i>=1; --i)
    {
        swap(a[1], a[i]);
        heapSize--;
        Downheap(a, heapSize, 1);
    }
    xuatMang(n, a);

    return 0;
}