TopK 问题

package p56;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.PriorityQueue;

/**
 * TopK 最小的k个数
 * @author Guozhu Zhu
 * @date 2019/4/13
 * @version 1.0
 *
 */
public class Solution01 {
	
	/* ========== Test ========== */
	public static void main(String[] args) {
		int[] arr = new int[] {1, 3, 2, 4, 6, 5, 9};
		ArrayList<Integer> res = method03(arr, 4);
		System.out.println(res);
	}
	
	//1. quickSort T(o) = nlogn
	public static ArrayList<Integer> method01(int[] arr, int k) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if (arr.length == 0 || k > arr.length) {
			return res;
		}
		quickSort(arr, 0, arr.length-1);
		for (int i = 0; i < k; i++) {
		    res.add(arr[i]);
		}
		return res;
	}
	
	public static void quickSort(int[] arr, int left, int right) {
		if (left > right) {
			return;
		}
		int i = left;
		int j = right;
		int k = arr[left];
		while (i < j) {
			while (i < j && arr[j] >= k) {
				j--;
			}
			arr[i] = arr[j];
			while (i < j && arr[i] <= k) {
				i++;
			}
			arr[j] = arr[i];
		}
		arr[i] = k;
		quickSort(arr, left, i-1);
		quickSort(arr, i+1, right);
	}
	
	//2. 部分排序, 冒泡, T(o) = n*k
	public static ArrayList<Integer> method02(int[] arr, int k) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if (arr.length == 0 || k > arr.length) {
			return res;
		}
		for (int i = 0; i < k; i++) {
			for (int j = 0; j < arr.length-1-i; j++) {
				if (arr[j+1] > arr[j]) {
					int temp = arr[j+1];
					arr[j+1] = arr[j];
					arr[j] = temp;
				}
			}
		}
		for (int i = arr.length-1; i > arr.length-1-k; i--) {
			res.add(arr[i]);
		}
		return res;
	}
	
	//3. 大顶堆, T(o) = nlogk
	public static ArrayList<Integer> method03(int[] arr, int k) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if (arr.length == 0 || k > arr.length) {
			return res;
		}
		PriorityQueue<Integer> maxHeap = new PriorityQueue<Integer>(new Comparator<Integer>() {

			@Override
			public int compare(Integer o1, Integer o2) {
				// TODO Auto-generated method stub
				return o2.compareTo(o1);
			}
		});
		for (int i = 0; i < arr.length; i++) {
			if (maxHeap.size() < k) {
				maxHeap.offer(arr[i]);
			} else if (maxHeap.peek() > arr[i]) {
				maxHeap.poll(); //最大的poll掉
				maxHeap.offer(arr[i]);
			}
		}
		for (Integer i : maxHeap) {
			res.add(i);
		}
		/*for (int i = 0; i < k; i++) {
			res.add(maxHeap.poll());
		}*/
	    Collections.reverse(res);
		return res;
	}

}

package p56;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.PriorityQueue;

/**
 * TopK 最大的k个数
 * @author Guozhu Zhu
 * @date 2019/4/13
 * @version 1.0
 *
 */
public class Solution02 {
	
	/* =========== Test ========== */
	public static void main(String[] args) {
		int[] arr = new int[] {1, 4, 3, 2, 9, 8, 0, 5};
		ArrayList<Integer> res = method03(arr, 4);
		System.out.println(res);
	}
	
	//1. 快速排序全排序
	public static ArrayList<Integer> method01(int[] arr, int k) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if (arr.length == 0 || k > arr.length) {
			return res;
		}
		quickSort(arr, 0, arr.length-1);
		for (int i = arr.length-1; i > arr.length-1-k; i--) {
			res.add(arr[i]);
		}
		return res;
	}
	
	public static void quickSort(int[] arr, int left, int right) {
		if (left > right) {
			return ;
		}
		int i = left;
		int j = right;
		int k = arr[i];
		while (i < j) {
			while (i < j && arr[j] >= k) {
				j--;
			}
			arr[i] = arr[j];
			while (i < j && arr[i] <= k) {
				i++;
			}
			arr[j] = arr[i];
		}
		arr[i] = k;
		quickSort(arr, left, i-1);
		quickSort(arr, i+1, right);
	}
	
	//2. 部分排序，
	public static ArrayList<Integer> method02(int[] arr, int k) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if (arr.length == 0 || k > arr.length) {
			return res;
		}
		for (int i = 0; i < k; i++) {
			for (int j = 0; j < arr.length-1-i; j++) {
				if (arr[j+1] < arr[j]) {
					int temp = arr[j+1];
					arr[j+1] = arr[j];
					arr[j] = temp;
				}
			}
		}
		for (int i = arr.length-1; i > arr.length-1-k; i--) {
			res.add(arr[i]);
		}
		return res;
	}
	
	//3. 堆排序
	public static ArrayList<Integer> method03(int[] arr, int k) {
		ArrayList<Integer> res = new ArrayList<Integer>();
		if (arr.length == 0 || k > arr.length) {
			return res;
		}
		PriorityQueue<Integer> minHeap = new PriorityQueue<Integer>(new Comparator<Integer>() {

			@Override
			public int compare(Integer o1, Integer o2) {
				// TODO Auto-generated method stub
				return o1.compareTo(o2);
			}
		});
		for (int i = 0; i < arr.length; i++) {
			if (minHeap.size() < k) {
				minHeap.offer(arr[i]);
			} else if (arr[i] > minHeap.peek()) {
				minHeap.poll();
				minHeap.offer(arr[i]);
			}
		}
        for (int i = 0; i < k; i++) {
        	res.add(0, minHeap.poll());
        }
        return res;
	}

}