Quick Sort - Time Complexity

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

int quicksort(int a[], int first, int last)
{
    int i, j, pivot, temp;

    if(first < last)
    {
        pivot = first;
        i = first;
        j = last;

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

int main()
{
    int i, num, a[270000];
    double start, end;
    srand(time(0));

    printf("Enter input size: ");
    scanf("%d", &num);

    printf("\nInput Array: ");
    for(i = 0; i < num; i++)
    {
        a[i] = rand()%1000;
        printf("%d ", a[i]);
    }

    start = clock();
    quicksort(a, 0, num-1);
    end = clock();

    printf("\nAfter sort: ");
    for(int i = 0; i < num; i++)
    {
        printf("%d ", a[i]);
    }
    printf("\n\nExecution time: %.3lf s\n\n", (end-start)/1000);
}