求第k大的数(用到快速排序算法的思想)

Posted 2020-06-22 沙漠孤舟

//下面两种part效率比较：相同运算量下part比part2快5倍左右，part2写法简单但是效率低

#include "stdafx.h"
#include <iostream>
#include <stdio.h>
using namespace std;

int part(int *arr, int l , int r)
{
    c_num += r - l;
    swap(arr[r],arr[l+rand()%(r-l)]);
    int q = r--;
    while( l < r)
    {
        while(l <= r && arr[l] <= arr[q]) ++l; 
        while(l < r && arr[r] >= arr[q]) --r;
        if(l<r&&arr[l]>arr[r]) swap(arr[l], arr[r]);
    }
    if(l<q&&arr[l]>arr[q]) swap(arr[l], arr[q]);    
    return l;
}

int part2(int *arr, int l , int r)
{
    c_num += r - l;
    int k, q = l;
    swap(arr[l], arr[l+rand()%(r-l)]);
    for(k = l+1; k <= r; ++k) 
    {
        if(arr[k] <= arr[q])
        {
            if(++l != k)
                swap(arr[l], arr[k]);
        }
    }
    swap(arr[l], arr[q]);    
    return l;
}

void qsort(int *arr, int l, int r)
{
    if(l >= r) return;
    int mid = part(arr, l , r);
    qsort(arr, l, mid-1);
    qsort(arr,mid+1, r);
}

int findNumK(int *arr, int l, int r, const int k)
{
    if(l >= r)
    {
        return arr[k];
    }
    int mid = part(arr, l , r);
    if(k < mid)
    {
        return findNumK(arr, l, mid-1, k);
    }
    else if (k > mid)
    {
        return findNumK(arr,mid+1, r, k);
    }
    else
    {
        return arr[k];
    }
}
#define MAX_RAND 82934829
int myRand(int n)
{
    return  (long((double)rand()/RAND_MAX * MAX_RAND))%n;
}
const int n = 10000000;
int ki = 100;
int arr[n];
int brr[n];
int _tmain(int argc, _TCHAR* argv[])
{
    for(int i = 0; i < n; ++i)
    {
        arr[i] = i;
    }
    for(int i = 0; i < n; ++i)
    {
        swap(arr[i],arr[myRand(n)]);
    }
    for(int i = 0; i < n; ++i)
    {
        brr[i] = arr[i];
    }
    printf("@@@@@@@\n");
    while(cin>>ki)
    {
        printf("\n%d %d\n", c_num, findNumK(brr, 0, n-1, n-ki));
        for(int i = 0; i < n; ++i)
        {
            brr[i] = arr[i];
        }
        c_num = 0;
    }
    printf("\n################\n");    
    c_num = 0;
    qsort(arr, 0, n-1);
    for(int i = n-1; i >= n - ki; --i)
    {        
        printf("%d ", arr[i]);
    }
    printf("\n%d\n",c_num);
    getchar();
    return 0;
}