Algorithm for grouping identical neighbors in a list

Question

I have a list that I want to reduce to a smaller list by grouping identical neighbors. This list has many many redundant entries.

Example list:

1,1,1,1,1,2,2,2,3,3,1,1,1,2,2,2,2,2,2,2,2,2,4,4,4,4,1,1,1,1,1,1

After 'compression'

1,2,3,1,2,4,1

Is there an algorithm for doing this that is faster than O(n)? In other words, faster than just looking at the next neighbor?

while (1)
    n = 0, m = 1
    if (list[n] == list[m])
        remove list[m]
    else 
        n = m
        m = m + 1
    if (m > list.size)
        break

If not, what about this problem makes it O(n)?