剑指Offer 最小的K个数

题目描述
输入n个整数，找出其中最小的K个数。例如输入4,5,1,6,2,7,3,8这8个数字，则最小的4个数字是1,2,3,4,。

1.利用Java的优先队列PriorityQueue（小顶堆）

import java.util.ArrayList;
import java.util.PriorityQueue;
public class Solution {
    public ArrayList<Integer> GetLeastNumbers_Solution(int [] arr, int k) {
        if(k > arr.length) {
            return new ArrayList<Integer>();
        }
        ArrayList<Integer> list = new ArrayList<Integer>();
        PriorityQueue<Integer> queue = new PriorityQueue<>();
        for(int i = 0; i < arr.length; i++) {
            queue.offer(arr[i]);
        }
        for(int i = 0; i < k; i++) {
            list.add(queue.poll());
        }
        return list;
    }
}

2.快排变形

import java.util.ArrayList;

public class Solution {

    private static int partition(int[] arr, int low, int high) {
        //确定边界值
        int pivot = arr[low];
        while (low < high) {
            //从右往左找，找到第一个小于边界值的数
            while (low < high && arr[high] >= pivot) {
                //大于区左移
                high--;
            }
            //赋值给小于区的数给小于区的最后一个数
            arr[low] = arr[high];
            //左往右找，找到一个大于边界值的数
            while (low < high && arr[low] <= pivot) {
                //小于区右移
                low++;
            }
            //将该值赋给大于区的第一个数
            arr[high] = arr[low];
        }
        //将边界值放回
        arr[low] = pivot;
        return low;
    }


    public static ArrayList<Integer> GetLeastNumbers_Solution(int [] arr, int k) {=
     if (arr == null || arr.length == 0 ||arr.length < k || k == 0) {
            return new ArrayList<>();
        }
        int low = 0;
        int high = arr.length - 1;
        int pos = partition(arr, low, high);
        while (low < high&&pos!=k-1) {
             pos = partition(arr, low, high);
            if (pos < k - 1) {
                low = pos + 1;
            } else {
                high = pos - 1;
            }
        }
    ArrayList<Integer> list = new ArrayList<>();
       for(int i =0;i<k;i++){
           list.add(arr[i]);
       }
        return list;
    }
}

3.自己构建小根堆：

import java.util.ArrayList;
import java.util.PriorityQueue;
public class Solution {
    public ArrayList<Integer> GetLeastNumbers_Solution(int [] arr, int k) {
            if (arr.length < k || k == 0) {
            return new ArrayList<>();
        }
        for(int i = arr.length/2-1;i>=0;i--){
            adjustHeap(arr,i,arr.length);
        }
        ArrayList<Integer>list = new ArrayList<Integer>(k);
        for(int j = arr.length-1;j>=arr.length-k;j--){
            swap(arr,0,j);
            list.add(arr[j]);
            adjustHeap(arr,0,j);
        }
        return list;
    }
   public   void swap(int[] arr , int i, int j){
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
    public void adjustHeap(int [] arr,int index,int heapSize){
        int temp = arr[index];
        for(int k = index*2+1;k<heapSize;k = k*2+1){
            if(k+1<heapSize&&arr[k]>arr[k+1]){
                k++;
            }
            if(arr[k]<temp){
                arr[index] = arr[k];
                    index = k;
            }else{
                break;
            }
        }
        arr[index] = temp;
    }
    
    
}