数据结构与算法分析1.1 选择问题

数据结构与算法分析-C语言实现，1.1题 实现选择问题，并打印不同N规模是运行时间：
用了自己实现的快速排序法，各个N规模显示如下：
root@ltegan:chapter1# ./a.out 
Before choose issue clock() returns: 27834 clocks-per-sec (0.03 sec)
times() yields: user CPU:0.02; system CPU:0.00
Your K[500000] max is :50 
After choose issue clock() returns: 12527475 clocks-per-sec (12.53 sec)
times() yields: user CPU:12.52; system CPU:0.00 <<<<大部分时间都是在跑用户空间的代码
N:1000000, clock:12499641(12.50 sec) <<<<<<<这里使用了12.5秒
Before choose issue clock() returns: 12527596 clocks-per-sec (12.53 sec)
times() yields: user CPU:12.52; system CPU:0.00
Your K[500] max is :49 
After choose issue clock() returns: 12527711 clocks-per-sec (12.53 sec)
times() yields: user CPU:12.52; system CPU:0.00
N:1000, clock:115(0.00 sec)<<<<<<<这里规模不大，1秒都不到
Before choose issue clock() returns: 13687813 clocks-per-sec (13.69 sec)
times() yields: user CPU:13.60; system CPU:0.08

Segmentation fault<<<<<<<这里因为规模太大，递归过头，段错误。N = 1 0000 0000;

root@ltegan:chapter1# 
//代码：
/*
* choose issue, there are N numbers, ensure the k-th max number
* 3 4 2 5, 3-th max number is 3
* sort the numbers big -> small, and return index k number
*/
#include "stdio.h"
#include <stdlib.h>
#include <assert.h>
#include <time.h>
#include <string.h>
#include <sys/times.h>
#include <unistd.h>

int find_mid(unsigned int * array, int start, int end)
{
int mid = start;
unsigned int tmp;
int i;
for (i = start+1; i <= end; i++) {
if (array[mid] < array[i]) {
tmp = array[mid];
array[mid] = array[i];
if (mid+1 == i) {
array[i] = tmp;
}
else {
array[i] = array[mid+1];
array[mid+1] = tmp;
}
mid++;
}
}
return mid;
}

int my_qsort(unsigned int * array, int start, int end)
{
if (start < end) {
int mid = find_mid(array, start, end);
my_qsort(array, start, mid-1);
my_qsort(array, mid+1, end);
}
return 0;
}

static clock_t 
displayProcessTimes(const char *msg)
{
struct tms t;
clock_t clockTime; 

static long clockTicks = 0;

if (msg != NULL) printf("%s", msg);
if (clockTicks == 0) {
clockTicks = sysconf(_SC_CLK_TCK);
if (clockTicks == -1) exit(-1); 
}
clockTime = clock();
if (clockTime == -1) exit(-1);
printf(" clock() returns: %ld clocks-per-sec (%.2f sec)\n",
(long)clockTime, (double)clockTime / CLOCKS_PER_SEC);

        //打印一下cpu用时和用户进程用时。
if (times(&t) == -1) exit(-1);
printf(" times() yields: user CPU:%.2f; system CPU:%.2f\n",
(double)t.tms_utime/clockTicks, (double)t.tms_stime/clockTicks);
return clockTime; 
}

void print_array(unsigned int * array, int a_len)
{
size_t i;
printf("========================\n");
for (i = 0; i < a_len; i++ ) {
printf(" %u", array[i]);
}
printf("\n########################\n");
}
unsigned int choose_issue(unsigned int * array, int a_len, int k)
{
//print_array(array, a_len);
my_qsort(array, 0, a_len-1);
//print_array(array, a_len);

return array[k-1];
}

unsigned int * full_array( int n )
{
int a_len, i;
unsigned int * array = NULL;
a_len = n * sizeof(unsigned int);
        array = malloc(a_len);
        if (array) {
                memset(array, 0, a_len);
                srand(time(NULL));
                for (i = 0; i < n; i++) {
                        array[i] = rand() % 100;
                }
        }
        else {
                assert(0);
        }

return array;
}

int main()
{
int N = 0;
clock_t startClock, stopClock;
unsigned int *array = NULL;
N = 1000000;
array = full_array( N );
startClock = displayProcessTimes("Before choose issue");
printf("Your K[%d] max is :%u \n", N/2, choose_issue(array, N, N/2));
stopClock = displayProcessTimes("After choose issue");
printf("N:%d, clock:%ld(%.2f sec)\n", N, (long)(stopClock - startClock), (double)(stopClock - startClock)/CLOCKS_PER_SEC);
if (array) free(array);

N = 1000;
array = full_array( N );
startClock = displayProcessTimes("Before choose issue");
printf("Your K[%d] max is :%u \n", N/2, choose_issue(array, N, N/2));
stopClock = displayProcessTimes("After choose issue");
printf("N:%d, clock:%ld(%.2f sec)\n", N, (long)(stopClock - startClock), (double)(stopClock - startClock)/CLOCKS_PER_SEC);
if (array) free(array);

N = 100000000;
array = full_array( N );
startClock = displayProcessTimes("Before choose issue");
printf("Your K[%d] max is :%u \n", N/2, choose_issue(array, N, N/2));
stopClock = displayProcessTimes("After choose issue");
printf("N:%d, clock:%ld(%.2f sec)\n", N, (long)(stopClock - startClock), (double)(stopClock - startClock)/CLOCKS_PER_SEC);
if (array) free(array);
}