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.  随机的调换数组     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.   先建立K的最大堆，K个值，那么起始点为K/2-1        int rootIndex=k/2-1;           while(rootIndex>=0){               reheap(heap,rootIndex,k-1);               rootIndex--;           }           for(int i=k,len=heap.length;i<len;i++){   //建立好K的最大堆之后，循环，将length-k之外的值不断的和最大值进行对比，小于最大值，就纳入堆中，调整堆的最大堆            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;       }   }