Why the sorted-ness matters

If the array were unsorted you'd reach for a hash set. But this one is sorted, so every group of equal values sits in a contiguous run — [1, 1, 2, 2, 2, 3]. That makes one-pass in-place de-duplication possible.

The two-pointer pattern

Use two indices:

• write points to where the next unique value will be stored.
• read walks through the array.

Initialize both at 0 (the first element is trivially unique). When a[read] differs from a[write], advance write and copy:

write = 0
for read in 1..n-1:
    if a[read] != a[write]:
        write += 1
        a[write] = a[read]
return write + 1

After the loop, the first write + 1 slots of a hold the unique values in their original order. Total work: O(n) time, O(1) extra memory.

Output format

Print the new length on line one, then the unique values space-separated on line two. If N is zero, print 0 on line one and an empty line two — but the tests don't include that case.