寻找最小的k个数___堆排序

在一场笔试中,碰到的题目,给一个数组,找出最小的K个数,当然首先想到的就是排序了,库函数sort一下,直接输出.

可以肯定的是这肯定不是出题者想看到的答案,各种排序算法中堆排序对这种寻找最大最小的K个数效率应该是最高的.

可惜的是笔试现场没能一挥而就...之后在电脑上实现了一下,完全的堆排序,对于寻找K个最大最小的只需用大根堆,小根堆然后判断一下到第K个就可以停止了.

#include <algorithm>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <cstdio>
#include <cstdlib>
#include <cctype>
#include <cmath>
#include <iomanip>
#include<windows.h>
using namespace std;
static int heapsize;

void maxheap(int a[],int i)
{
    //adapt down
    int leftchild=2*i;
    int rightchild=2*i+1;
    int largest=-1;
    if(leftchild<=heapsize&&a[i]<a[leftchild])
        largest=leftchild;
    else
        largest=i;
    if(rightchild<=heapsize&&a[largest]<a[rightchild])
        largest=rightchild;
    if(largest!=i)
    {
        swap(a[i],a[largest]);
        maxheap(a,largest);
    }
}

void buildheap(int a[])
{
    // build-heap
    for(int i=heapsize/2; i>=1; --i)
    {
        maxheap(a,i);
    }
}

void heapsort()
{
    int a[21];
    //Seed the random-number generator with current time
    srand(time(NULL));
    for(int i=1; i<=20; ++i)
    {
        a[i]= rand()%99;
    }
    //build the MAX-HEAP
    buildheap(a);

    //do sort
    for(int i=20; i>=2; --i)
    {
        //swap the largest and the last
        swap(a[i],a[1]);
        //Remove the largest
        heapsize--;
        //adapt from a[1]
        maxheap(a,1);
    }
    for(int i=1; i<=20; ++i)
    {
        cout<<a[i]<<" ";
    }
    cout<<endl;
}


int main()
{
    int k=10;
    while(k>0)
    {
        heapsize=20;
        heapsort();
        --k;
        //Wait 1s is to ensure that to produce time-based random number
        Sleep(1000);

    }
    return 0;
}

结果: