Hi, here's your problem today. This problem was recently asked by LinkedIn:

You are given a positive integer N which represents the number of steps in a staircase. You can either climb 1 or 2 steps at a time. Write a function that returns the number of unique ways to climb the stairs.

```
def staircase(n):
# Fill this in.
print staircase(4)
# 5
print staircase(5)
# 8
```

Can you find a solution in O(n) time?