Advanced · Recursion · 166 of 264
Climbing stairs
You climb a staircase taking 1, 2 or 3 steps at a time. In how many different ways can you reach the top of n steps? Use memoisation.
Input: n (1 to 60). Output: the number of ways.
Examples
Input
3
Output
4
Input
1
Output
1
Input
4
Output
7
2 more tests check other inputs when you press Check.
Stuck? The idea behind recursion
A recursive function calls itself on a smaller version of the problem, until it reaches a case small enough to answer directly (the base case).
Trust the smaller call to be right, and only work out how to build this answer from it.
def factorial(n):
if n == 0:
return 1
return n * factorial(n - 1)
print(factorial(5))Watch for: Without a base case, or if the problem doesn't get smaller, it never stops: Python gives up with a RecursionError.