Minor changes in indexing take twice memory and time - c

Question

I submitted two slightly different solutions for a problem in which you have remove duplicate elements of an array, so that the relative order does not change.
In first method, I take input sequentially, and store in an array sequentially, and then two nested loops, to check for each element in input array, if it is unique, I store it in another array.
While in second method, I take input sequentially, but store then in reverse order. Rest is same, except the required indexing/iterator changes.
The second one takes twice as much time and memory as first.
Though, the difference is very minor, then why is there a huge difference in time?
Or How can I check exactly why is there this difference?
Actual Problem: You are given an array a consisting of n integers. Your task is to make these array unique by removing duplicates from them.
You have to leave only the rightmost occurrence for each element of the array and remove all other occurrences. The relative order of the remaining unique elements should not be changed.
First method (15 ms, 4 KB):
#include <stdio.h>
int main(void)
{
int n, i, c;
scanf("%i", &n);
int arr[n], nondupe[n];
for (i = 0; i < n; i++)
scanf("%i", arr + i); // Store sequentially
int k = n - 1;
nondupe[k--] = arr[n - 1]; // add last element in non-duplicate array (because rightmost entries are required)
for (i = n - 1; i > 0; i--)
for (c = n - 1; c > k; c--) // Nested loops to iterate over all elements in both arrays
{
if (arr[i - 1] == nondupe[c])
break;
if (c == k + 1)
nondupe[k--] = arr[i - 1];
}
printf("%i\n", n - k - 1);
for (i = k + 1; i < n; i++)
printf("%i ", nondupe[i]);
}
Second (30 ms, 8 KB):
#include <stdio.h>
int main(void)
{
int n, i, c;
scanf("%i", &n);
int arr[n], nondupe[n];
for (i = n; i > 0; i--)
scanf("%i", arr + i - 1);
int k = 0;
nondupe[k++] = arr[0];
for (i = 0; i < n; i++)
for (c = 0; c < k; c++)
{
if (arr[i] == nondupe[c])
break;
if (c == k - 1)
nondupe[k++] = arr[i];
}
printf("%i\n", k);
for (i = k; i > 0; i--)
printf("%i ", nondupe[i - 1]);
}