Technically the array doesn't need to be sorted, since the condition was for a[i] != a[i-1], not a[i] > a[i-1], meaning that same numbers had to be grouped next to each other. Something like 1 1 1 5 5 3 3 3 2 2 9 9 7 7 7, which you can easily count the number of marked elements for.

For arrays in which same numbers aren't grouped together, for example like 1 3 5 2 3 1 4 3 1 5, you can just treat every element as a different group