求数组中的K大数,快排思想(Java改…

最新推荐文章于 2020-12-24 13:40:22 发布

CHINAonlyQiu

最新推荐文章于 2020-12-24 13:40:22 发布

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分类专栏： JavaSE程序

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本文介绍了一种通过随机化分区算法寻找数组中第K大的元素的方法。该算法首先随机选择一个基准元素进行分区，然后根据分区结果递归地在数组的不同部分继续搜索，直至找到第K大的元素。

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package cn.edu.nwsuaf.cie.qhs;

import java.util.Random;
import java.util.Scanner;
//
// *
// * @author 静寞小森(沧海森林)
// * Our goal is to get the number which is bigger than other k.
// * So we program this program to reach our goal.
// *
//
public class GetGreateK {

private int initArray[];
public int[] getInitArray() {
return initArray;
}
public void setInitArray(int[] initArray) {
this.initArray = initArray;
}
public GetGreateK(){}
public GetGreateK(int[] array){
this.initArray = array;
}
public int random_partion(int start,int length){
Random rand = new Random();
int index = rand.nextInt(length)+start;
int i = start-1;
int j = start;
System.out.println("index="+index+";start="+start+";length="+length);
this.swap(index, start+length-1);
for(j=start;j<start+length;j++){
if(initArray[j] < initArray[start+length-1]){
this.swap(++i, j);
System.out.println("i="+i+";j="+j);
}
}
this.swap(++i, start+length-1);
System.out.println("i="+i+";start+length-1="+(start+length-1));
// System.out.println("------>"+i);
return i;
}
public int getMaxK(int start,int length,int k){
int mid;
if (k<=0) return -1;
if (length<k) return -2;
mid = this.random_partion(start, length);
if ((mid-start) == length-k){System.out.println("----->this is the first!!!!!"); return initArray[mid];}
else if((mid-start) < length-k){System.out.println("----->this is the second!!!!!"); return getMaxK(mid+1,length-mid+start-1,k);}
else{System.out.println("----->this is the third!!!!!"); return getMaxK(start,mid-start,k-(length-mid+start));}
}

public void swap(int a,int b){
int temp = initArray[a];
initArray[a] = initArray[b];
initArray[b] = temp;
}

public static void main(String[] args) {
// TODO Auto-generated method stub
//12012 3 945 965 66 232 65 7 8 898 56 878 170 13 5
int[] array = {12012,3,945,965,66,232,65,7,8,898,56,878,170,13,5};
int length;
int K = 9;
// Scanner scanner = new Scanner(System.in);
// System.out.println("请输入数组长度：");
// length = scanner.nextInt();
// array = new int[length];
// for(int i = 0; i < length;i++){
// System.out.println("请输入第"+(i+1)+"个数：");
// array[i] = scanner.nextInt();
// }
// System.out.println("请输入K：");
// K = scanner.nextInt();
GetGreateK kth = new GetGreateK();
kth.setInitArray(array);
int maxK = kth.getMaxK(0,array.length,K);
System.out.println("maxK---->"+maxK);
}

}