题目描述

寻找N个数字中最大的K个数字

package chapter_2_binary;

import java.util.Random;
import java.util.Scanner;


/**
 * @author LiangGe
 * 
 * 寻找一批数字中最K大数
 * 
 * 使用方法类似快速排序查找
 * 
 */
public class question_2_5_1 {
	private static Random random = new Random(47);
	
	public static void swap(int[] data, int i, int j) {
		int tmp = data[i];
		data[i] = data[j];
		data[j] = tmp;
	}
	
	/**
	 * @param data
	 * @param start
	 * @param end
	 * @return
	 * 
	 * 随机版本的划分法
	 * 
	 */
	public static int partition(int[] data, int start, int end) {
		//　每次随机查收划分基准的位置，并与末尾元素交换
		int shift = random.nextInt(end - start + 1);
		int p = start + shift;
		swap(data, p, end);
		int tmp = data[end];
		// 此处容易出现，i误以为每次都是-1
		int i = start - 1;
		// j从start开始，误以为-1
		for(int j=start; j<=end; j++) {
			if(data[j] < tmp) {
				i ++;
				swap(data, j, i);
			}
		}
		swap(data, i+1, end);
		return i + 1;
	}
	
	public static int findKBigest(int[] data, int k, int start, int end) {
		int partition = partition(data, start, end);
		int tmp = end - partition;
		// 前两个if出现的概率最大，放在前面，减少if的判断次数
		if(k < tmp) {
			return findKBigest(data, k, partition + 1, end); 
		} else if(k > (tmp + 1)) {
			return findKBigest(data, k - (tmp + 1), start, partition - 1);
		} else if(k == tmp) {
			return partition + 1;
		} else {
			return partition;
		}
	}
	
	/**
	 * @param data
	 * @param k
	 * @param start
	 * @param end
	 * @return
	 * 
	 * 使用类似快速排序算法寻找前K大的数字
	 * 
	 */
	public static int findKBig(int[] data, int k, int start, int end) {
		if(k > data.length) {
			return 0;
		} else if(k < 0) {
			return end + 1;
		} else {
			return findKBigest(data, k, start, end);
		}
	}
	
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in); 
		int num = scanner.nextInt();
		scanner.nextLine();
		
		while(num -- > 0) {
			int n = scanner.nextInt();
			int k = scanner.nextInt();
			
			scanner.nextLine();
			String string = scanner.nextLine();
			String[] strs = string.split(" ");
			int[] data = new int[strs.length];
			
			for(int i=0; i< data.length; i++) {
				data[i] = Integer.parseInt(strs[i]);
			}
			
			int start = findKBig(data, k, 0, data.length-1);
			
			for(int i=start; i< data.length; i++) {
				System.out.printf("%3d", data[i]);
			}
		}
	}
}