java-5.查找最小的K个元素-使用最大堆

import java.util.Arrays;
import java.util.Random;


public class MinKElement {

	/**
	 * 5.最小的K个元素
	 * I would like to use MaxHeap.
	 * using QuickSort is also OK
	 */
	public static void main(String[] args) {
		MinKElement mke=new MinKElement();
		int[] a={1,2,3,4,5,6,7,8};
		int k=4;
		mke.disArrange(a);
		System.out.println("after disarranging,the array a[]="+Arrays.toString(a));
		mke.findKMin(a,k);
		
	}

	//rearrange the array.just for test.
	public void disArrange(int[] a){
		for(int i=0,len=a.length;i<len;i++){
			Random random=new Random();
			int j=random.nextInt(len);
			swap(a,i,j);
		}
	}
	public void findKMin(int[] a,int k){
		int[] heap=a;//you can do this:int[] heap=new int[k].but that maybe space-cost
		
		//create MaxHeap of K elements.from the lastRootIndex to 0.
		int rootIndex=k/2-1;
		while(rootIndex>=0){
			reheap(heap,rootIndex,k-1);
			rootIndex--;
		}
		for(int i=k,len=heap.length;i<len;i++){
			if(heap[i]<heap[0]){
				heap[0]=heap[i];
				reheap(heap,0,k-1);  
			}
		}
		System.out.print("the K min elements=");
		for(int i=0;i<k;i++){
			System.out.print(heap[i]+",");
		}
	}
	
	//reheap:from root to lastIndex.
	public void reheap(int[] heap,int rootIndex,int lastIndex){
		int orphan=heap[rootIndex];
		boolean done=false;
		int leftIndex=rootIndex*2+1;
		while(!done&&leftIndex<=lastIndex){
			int largerIndex=leftIndex;
			if(leftIndex+1<=lastIndex){
				int rightIndex=leftIndex+1;
				if(heap[rightIndex]>heap[leftIndex]){
					largerIndex=rightIndex;
				}
			}
			//Attention! should not use -->heap[root]<heap[largerIndex]<--.
			//I spend time to find the problem....
			if(orphan<heap[largerIndex]){
				heap[rootIndex]=heap[largerIndex];
				rootIndex=largerIndex;
				leftIndex=rootIndex*2+1;
			}else{
				done=true;
			}
		}
		heap[rootIndex]=orphan;
		
	}
	public void swap(int[] a,int i,int j){
		int temp=a[i];
		a[i]=a[j];
		a[j]=temp;
	}
}