C randomized high-speed sorting (improved split function)

Question

I am a computer science student (I just started), I worked on writing a randomized consolidated version of Quicksort from the pseudo-code. I wrote and tested it and it all works great ...

Part of the section looks a little more complicated, as it seems that I missed something or changed my mind. I cannot understand if this is good, or if I made some possible mistakes.

In short: it works, but how to do better?

Thank you in advance for your help.

void partition(int a[],int start,int end)
{
    srand (time(NULL));
    int pivotpos = 3;   //start + rand() % (end-start);
    int i = start;    // index 1
    int j = end;      // index 2
    int flag = 1;
    int pivot = a[pivotpos];   // sets the pivot value
    while(i<j && flag)      // main loop
    {
        flag = 0;
        while (a[i]<pivot)
        {
            i++;
        }
        while (a[j]>pivot)
        {
            j--;
        }
        if(a[i]>a[j]) // swap && sets new pivot, and restores the flag
        {
            swap(&a[i],&a[j]);
            if(pivotpos == i)
                pivotpos = j;
            else if(pivotpos == j)
                pivotpos = i;
            flag++;
        }
        else if(a[i] == a[j])       // avoids getting suck on a mirror of values (fx pivot on pos 3 of : 1-0-0-1-1)
        {
            if(pivotpos == i) 
                j--;
            else if(pivotpos == j)
                i++;
            else
            {
                i++;
                j--;
            }
            flag++;
        }
    }
}

Answer 1

partition() , , .

PARTITION(A, p, r)
1 x ← A[r]
2 i ← p - 1
3 for j ← p to r - 1
4   do if A[j] ≤ x
5       then i ←i + 1
6           exchange A[i] ↔ A[j]
7 exchange A[i + 1] ↔ A[r]
8 return i +1

- . , srand() partition.

// srand(time(NULL));
int partition(int* arr, int start, int end)
{
    int pivot_index = start + rand() % (end - start + 1);
    int pivot = arr[pivot_index ];

    swap(&arr[pivot_index ], &arr[end]); // swap random pivot to end.
    pivot_index = end;
    int i = start -1;

    for(int j = start; j <= end - 1; j++)
    {
        if(arr[j] <= pivot)
        {
            i++;
            swap(&arr[i], &arr[j]);
        }
    }
    swap(&arr[i + 1], &arr[pivot_index]); // place the pivot to right place

    return i + 1;
}

, , , :

Hoare-Partition(A, p, r)
x = A[p]
i = p - 1
j = r + 1
while true
    repeat
        j = j - 1
    until A[j] <= x
    repeat
        i = i + 1
    until A[i] >= x
    if i < j
        swap( A[i], A[j] )
    else
        return j

A [p... j] ≤ A [j + 1... r]. , quicksort :

QUICKSORT (A, p, r)
if p < r then
 q = Hoare-Partition(A, p, r)
 QUICKSORT(A, p, q)
 QUICKSORT(A, q+1, r)

Answer 2

quicksort, , , , . :

• Squeeze - , , . , ( ), ...
• Sweep - ( ) , , . , .

Sweep , quicksort , . , . , swap(), , temp-.

. , , , , quicksort , .

, , C/++ , "" . . quicksort(), .

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

void swap(int *lhs, int *rhs)
{
    if (lhs == rhs)
        return;

    int tmp = *lhs;
    *lhs = *rhs;
    *rhs = tmp;
}

int partition(int ar[], int len)
{
    int i, pvt=0;

    // swap random slot selection to end.
    //  ar[len-1] will hold the pivot value.
    swap(ar + (rand() % len), ar+(len-1));
    for (i=0; i<len; ++i)
    {
        if (ar[i] < ar[len-1])
            swap(ar + i, ar + pvt++);
    }

    // swap the pivot value into position
    swap(ar+pvt, ar+(len-1));
    return pvt;
}

void quicksort(int ar[], int len)
{
    if (len < 2)
        return;

    int pvt = partition(ar, len);
    quicksort(ar, pvt++); // note increment. skips pivot slot
    quicksort(ar+pvt, len-pvt);
}


int main()
{
    srand((unsigned int)time(NULL));

    const int N = 20;
    int data[N];

    for (int i=0; i<N; ++i)
    {
        data[i] = rand() % 50 + 1;
        printf("%d ", data[i]);
    }
    puts("");

    quicksort(data, N);

    for (int i=0; i<N; ++i)
        printf("%d ", data[i]);

    puts("");

    return 0;
}

(, )

32 49 42 49 5 18 41 48 22 33 40 27 12 47 41 6 50 27 8 7 
5 6 7 8 12 18 22 27 27 32 33 40 41 41 42 47 48 49 49 50 

: rand() % len, , , . , . quicksort , .