利用快速排序原理找出数组中前n大的数

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#define MAX_SIZE 400001
// 生成不重复的随机数序列写入文件
void gen_test_data(uint32_t cnt)
{
if( cnt >= MAX_SIZE){printf("cnt too largr\n");return;}
uint32_t i = 0;
char buf[MAX_SIZE];
for(;i < cnt;++i){buf[i] = 1;}
uint32_t n = 0;
char file_name[256];
snprintf(file_name,256,"test_data_%d.txt",cnt);
FILE *fp = fopen(file_name,"w");
if(NULL == fp){printf("open %s error!\n",file_name);return;}
while(n < cnt)
{
int32_t nRand = rand() % cnt;
while(buf[nRand] == 0)nRand = (nRand + 1)%cnt;
buf[nRand] = 0;
fprintf(fp,"%d ",nRand);
++n;
}
fclose(fp);
}

// 读取文件
void read_data(int32_t arr[],const uint32_t size,uint32_t *cnt,const int32_t data_cnt)
{
FILE *fp = NULL;
char file_name[256];
if(data_cnt > size){printf("data_cnt too largr\n");return;}
snprintf(file_name,256,"test_data_%d.txt",data_cnt);
fp = fopen(file_name,"r");
if(NULL == fp){printf("open %s error!\n",file_name);return;}
while(!feof(fp) && *cnt < size)
{
fscanf(fp,"%d ",&arr[*cnt]);
(*cnt)++;
}
fclose(fp);
}

// 快速排序
void quick_sort(int32_t arr[],int32_t low,int32_t high)
{
if(low >= high)return;
int32_t i = low,j = high,tmp = arr[i];
while(i<j)
{
while(i<j && arr[j] <= tmp)j--;
if(i<j){arr[i] = arr[j];i++;}
while(i<j && arr[i] > tmp)i++;
if(i<j){arr[j] = arr[i];j--;}
}
arr[i] = tmp;
quick_sort(arr,low,i-1);
quick_sort(arr,i+1,high);
}

// 找出最大n个数
void get_topn(int32_t arr[],int32_t low,int32_t high,const int32_t topn)
{
if(low >= high || topn > high)return;
int32_t i = low,j = high,tmp = arr[i];
while(i<j)
{
while(i<j && arr[j] < tmp)j--;
if(i<j)arr[i++] = arr[j];
while(i<j && arr[i] >= tmp)i++;
if(i<j)arr[j--] = arr[i];
}
arr[i] = tmp;
int32_t n = i - low + 1;
if (n == topn)return;
else if (n > topn)
get_topn(arr, low, i-1, topn);
else if (n < topn)
get_topn(arr, i+1, high, topn - n);
}

void dump(int32_t arr[],const uint32_t cnt)
{
uint32_t i = 0;
for(;i < cnt;++i)
{
printf("%6d ",arr[i]);
}
printf("\n");
}

int32_t main(int32_t argc,char *argv[])
{
int32_t data_cnt = 40000,top = 500;
int32_t arr[MAX_SIZE];
uint32_t cnt = 0;
gen_test_data(data_cnt);
read_data(arr,MAX_SIZE,&cnt,data_cnt);
get_topn(arr,0,cnt-1,top);
quick_sort(arr,0,top-1);
dump(arr,top);
//quick_sort(arr,0,cnt);
//dump(arr,cnt);
return 0;
}