Two passes: count first, then ask

Some questions can only be answered after seeing the whole list: "which is the first element that appears only once?" Looking at the first element you cannot know whether it repeats later.

1count = {}
2for v in values:
3    count[v] = count.get(v, 0) + 1
4for i, v in enumerate(values):
5    if count[v] == 1:
6        return i
7return -1

The first pass counts; the second walks the list again in its own order and asks the counts. Two passes are still 2n reads, not n × n.

It matters that the second pass runs over the list and not over the dictionary: the word "first" means the order in the list.

Tasks