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;

}