体验复杂度—两种排序算法的运行时间

/*
*Copyright (c) 2014,烟台大学计算机控制工程学院
*All right reserved
*文件名称：test.cpp
*作    者：邱暖
*完成日期：2015年9月14日
*版本号 ：v1.10
*问题描述：感受复杂度为O(n^2)的选择排序和复杂度为O(nlogn)的快速排序在运行时间上的差异
*/

（1）复杂度是O（n^2）的选择排序程序

#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#define MAXNUM 100000
void selectsort(int a[], int n)
{
        int i, j, k, tmp;
        for(i = 0; i < n-1; i++)
        {
                k = i;
                for(j = i+1; j < n; j++)
                {
                        if(a[j] < a[k])
                                k = j;
                }
                if(k != j)
                {
                        tmp = a[i];
                        a[i] = a[k];
                        a[k] = tmp;
                }
        }
}

int main()
{
    int x[MAXNUM];
    int n = 0;
    double t1,t2;
    FILE *fp;
    fp = fopen("numbers.txt", "r");
    if(fp==NULL)
    {
        printf("打开文件错！请下载文件，并将之复制到与源程序文件同一文件夹下。\n");
        exit(1);
    }
    while(fscanf(fp, "%d", &x[n])!=EOF)
        n++;
    printf("数据量：%d, 开始排序....", n);
    t1=time(0);
    selectsort(x, n);
    t2=time(0);
    printf("用时 %d 秒!", (int)(t2-t1));
    fclose(fp);
    return 0;
}

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff">运行结果：</span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff"><img src="http://img.blog.csdn.net/20150914170819324?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center" width="367" height="192" alt="" /></span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff">（2）复杂度为O（nlogn）的快速排序程序</span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff"></span><pre class="cpp" name="code">#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#define MAXNUM 100000
void quicksort(int data[],int first,int last)
{
    int i, j, t, base;
    if (first>last)
        return;
    base=data[first]; 
    i=first;
    j=last;
    while(i!=j) 
    {
        while(data[j]>=base && i<j) 
            j--;
        while(data[i]<=base && i<j) 
            i++;
        /*交换两个数*/
        if(i<j)
        {
            t=data[i];
            data[i]=data[j];
            data[j]=t;
        }
    }
    data[first]=data[i]; 
    data[i]=base; 
    quicksort(data,first,i-1);
    quicksort(data,i+1,last); 
}

int main()
{
    int x[MAXNUM];
    int n = 0;
    double t1,t2;
    FILE *fp;
    fp = fopen("numbers.txt", "r");
    if(fp==NULL)
    {
        printf("打开文件错！请下载文件，并将之复制到与源程序文件同一文件夹下。\n");
        exit(1);
    }    
    while(fscanf(fp, "%d", &x[n])!=EOF)
        n++;
    printf("数据量：%d, 开始排序....", n);
    t1=time(0);
    quicksort(x, 0, n-1);
    t2=time(0);
    printf("用时 %d 秒!", (int)(t2-t1));
    fclose(fp);
    return 0;
}

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff">运行结果：</span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff"><img src="http://img.blog.csdn.net/20150914171313847?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center" width="375" height="226" alt="" /></span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff">学习心得：</span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff">          通过对比两种程序的运行结果可以看出，复杂度是O（n^2）的选择排序的运行时间</span>

<span style="font-family:Arial;BACKGROUND-COLOR: #ffffff">比复杂度是O(nlogn)d的快速排序的运行时间要长。因此，我们不仅可以得到复杂度与运行时间成正比。</span>

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