Do you write as you go along or plan the story first?

The story unfolds as I write it. So my first chapter was pretty much as I first saw it. Sometimes the ideas come too fast for me to write them out so then I use bullets so that I don't forget and flesh them out later.

The ending took me a long time to figure out. So for ages and ages the story was 3/4s done and it frustrated me no end wondering how the story would conclude.