快速排序-《算法导论》学习笔记七

快排利用分治的思想，将问题规模切分为小的子规模问题，类似归并排序，归并是将两个待排序的子数组合并为一个有序的数组，而快排是将数组以某个数组值划分为两个子数组，一个数组的所有数比这个值小，一个数组的所有数比这个值大，然后再递归地对两个子数组再以某个数组值做切分和交换：

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <time.h>
#include <stdlib.h>


int partition( int *arr, int p, int r )
{
    int x = arr[r];
    int i = p - 1;
    int j = 0;
    int temp = 0;

    for ( j = p; j < r; j++ ) {
        if ( arr[j] <= x ) {
            i += 1;
            temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }
    }
    temp = arr[i + 1];
    arr[i + 1] = arr[r];
    arr[r] = temp;


    return i + 1;
}

void quick_sort( int *arr, int p, int r )
{
    int q = 0;

    if ( p < r ) {
        q = partition( arr, p, r );

        //printf("p->%d, q->%d, r->%d\n", p, q, r);

        quick_sort( arr, p, q - 1 );

        quick_sort( arr, q + 1, r );
    }
}

void check_is_inc_arr( 
    int *arr, 
    int len )
{
    int i = 0;

    for ( i = 0; i < len; i++ ) {
        if ( arr[i] < arr[i - 1] ) {
            printf("check_is_inc_arr fail.\n");
            return;
        }   
    }
}
//初始化随机数组
void initArr( int *arr, int lowV,
    int upV, int len )
{
    int i = 0;
    int size = upV - lowV;

    for ( i = 0; i < len; i++ )
    {
        arr[i] = rand() % size + lowV;
    }
}
void print_arr( 
    int *arr, 
    int len )
{
    int i = 0;

    printf("\n=============================================\n");
    for ( i = 0; i < len; i++ ) {
        printf("%d ", arr[i]);
    }
    printf("\n=============================================\n");
}
int main(
    int argc,
    char **argv )
{
    if ( argc != 4 ) {
        printf("input array length and the random value range.\n");
        exit( 0 );
    }

    srand( (int)time(NULL) );

    int length = atoi( argv[1] );
    int lowV = atoi( argv[2] );
    int upV = atoi( argv[3] );

    int *arr = ( int * )malloc( sizeof(int) * (length) );

    initArr( arr, lowV, upV, length );

    print_arr( arr, length );
    quick_sort( arr, 0, length - 1 );
    print_arr( arr, length );

    check_is_inc_arr( arr, length );

    return 0;
}