3.1.3 Quick sort, Merge sort, C (gcc)

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

void merge(int arr[], int l, int m, int r)
{
    int i, j, k;
    int n1 = m - l + 1;
    int n2 = r - m;

int L[n1], R[n2];

for (i = 0; i < n1; i++)
        L[i] = arr[l + i];
    for (j = 0; j < n2; j++)
        R[j] = arr[m + 1 + j];

i = 0;
    j = 0;
    k = l;
    while (i < n1 && j < n2)
    {
        if (L[i] <= R[j])
        {
            arr[k] = L[i];
            i++;
        }
        else
        {
            arr[k] = R[j];
            j++;
        }
        k++;
    }

while (i < n1)
    {
        arr[k] = L[i];
        i++;
        k++;
    }
    while (j < n2)
    {
        arr[k] = R[j];
        j++;
        k++;
    }
}

void mergeSort(int arr[], int l, int r)
{
    if (l < r)
    {
        int m = l + (r - l) / 2;
        mergeSort(arr, l, m);
        mergeSort(arr, m + 1, r);

merge(arr, l, m, r);
    }
}

void quicksort(int number[], int first, int last)
{
    int i, j, pivot, temp;
    if (first < last)
    {
        pivot = first;
        i = first;
        j = last;
        while (i < j)
        {
            while (number[i] <= number[pivot] && i < last)
                i++;
            while (number[j] > number[pivot])
                j--;
            if (i < j)
            {
                temp = number[i];
                number[i] = number[j];
                number[j] = temp;
            }
        }
        temp = number[pivot];
        number[pivot] = number[j];
        number[j] = temp;
        quicksort(number, first, j - 1);
        quicksort(number, j + 1, last);
    }
}

void sequentially(int arr[], int n)
{
    quicksort(arr, 0, n);
    mergeSort(arr, 0, n);
}

int main()
{
    int n;
    scanf("%d", &n);
    int arr[n];
    for (int i = 0; i < n; i++)
    {
        arr[i] = rand() % 1000;
    }
    double start = omp_get_wtime();
    sequentially(arr, n);
    double end = omp_get_wtime();
    printf("Sequential Time %lf \n", end - start);
    start = omp_get_wtime();
#pragma omp parallel
    {
#pragma omp sections
        {
#pragma omp section
            {
                mergeSort(arr, 0, n);
            }
#pragma omp section
            {
                quicksort(arr, 0, n);
            }
        }
    }
    end = omp_get_wtime();
    printf("Parallel Time %lf \n", end - start);
}

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Compilation time: 0.22 sec, absolute running time: 0.16 sec, cpu time: 0.09 sec, memory peak: 5 Mb, absolute service time: 0,49 sec