Given the following examples: arr[] = {-3, -2, -1, 0, 1, 2} → Ascending order arr[] = {2, 1, 0, -1, -2, -3} → Descending order arr[] = {2, 1, -3, 0…

One pass: the array is ascending if no element is smaller than the one before it, descending if none is larger; otherwise unordered. O(n) time, O(1) extra space.

#include <stdio.h>

int main(void) {
    int n;
    scanf("%d", &n);
    int a[n];
    for (int i = 0; i < n; i++)
        scanf("%d", &a[i]);

    int ascending = 1, descending = 1;
    for (int i = 1; i < n; i++) {
        if (a[i] < a[i - 1]) ascending = 0;
        if (a[i] > a[i - 1]) descending = 0;
    }

    if (ascending)       printf("Ascending order\n");
    else if (descending) printf("Descending order\n");
    else                 printf("Unordered\n");
    return 0;
}

{-3, -2, -1, 0, 1, 2} → Ascending; {2, 1, 0, -1, -2, -3} → Descending; {2, 1, -3, 0, -2, -1} → Unordered (it falls, then rises at -3 → 0).

— rhs