利用快速排序原理找出数组中前n大的数

Posted 2020-07-30 阿汤的博客

#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;
}