1. Given a staircase of n steps. You can climb in steps of 1, 2 and 3.
In how many ways can you climb the staircase?

Ex:
For 4 steps:
    1+1+1+1
    1+1+2
    1+2+1
    2+1+1
    1+3
    3+1
    2+2
So answer: 7