建堆，堆排序，堆维护

#include <iostream>

using namespace std;
//num is the number of the array,index from 0 to num-1
//the index relationship : child index is k then parent index is (k-2)/2
void shiftDown(int * an, int i, int num)
{
 	if(NULL == an || i < 0 || num < 0) return;
 	int temp = an[i];
	while(2*i+1 < num)
	{
		int index = 2 * i + 1;
		if(index+1 < num)
		{
			if(an[index] < an[index+1])
				index++;
		}
		if(temp < an[index])
		{
			an[i] = an[index];
			i     = index;
		}
		else break;
	}
	an[i] = temp;
}
void heapSort(int * an, int num) // heapSort is O(nlogn)
{
	for(int i = (num-2)/2; i >= 0; --i) //build heap O(n)
		shiftDown(an, i, num);
	for(int j = num-1; j >= 0; --j) //sort n*log(n) 
	{
		int temp = an[0];
		an[0] = an[j];
		an[j] = temp;
		shiftDown(an, 0, j);
	}
}
void shiftUp(int * an, int newIndex)
{
	if(NULL == an || newIndex < 0) return;
	int temp = an[newIndex];
	int parent = (newIndex-1)/2;
	while( parent >= 0)
	{
		if(temp > an[parent])
		{
			an[newIndex] = an[parent];
			newIndex = parent;
			parent = (newIndex-1)/2;
		}
		else break;
	}
	an[newIndex] = temp;
}
int main()
{
	int N;
	while(cin >> N)
	{
		int * arr = new int[N];
		for(int i = 0; i < N; ++i)
			cin >> arr[i];
		heapSort(arr, N);
		for(int j = 1; j <= 4; ++j)
			cout << arr[N-j] << endl;
		delete arr;
	}
	return 0;
}