There was no one viking hairstyle but from the archeological records we can assume that raiders generally kept their hairs and beards more closely trimmed to prevent grabbing in battle. There are numerous statuettes and depictions of vikings in armor who kept trimmed beards. There are also a certain number of mentions of what can mostly be described as a "reverse mullet", hair kept long and swept forward on top with a shaved neck. For a visual reference, see Björn in Historys Vikings.

We also know that Vikings in general were more concious about their hygiene and grooming than people of the era in general. They would bathe, comb their hairs and beards and change their clothes more frequently.

EDIT: Pliny the Elder - Historia Naturalis Book 28, p 191

Williams, M - Social Scandinavia in the Viking Age