Why is my solution on LeetCode not working (recursion, python)?

Question

class Solution:

    def climbStairs(self, n):
        """
        :type n: int
        :rtype: int
        """
        if n == 1:
            return 1
        if n == 2:
            return 2
        else:
            return self.climbStairs(n - 1) + self.climbStairs(n - 2)

My solution however is not working on Leetcode. Do you know why this is? Thank you so much!

I am trying to solve fibonacci sequence

Answer 1

You've chosen a poor algorithm. It's not that it's recursive, just horribly inefficient. Here's a more efficient recursive approach:

def climbStairs(self, n, res=1, nxt=1):
    if n == 0:
        return res
    return self.climbStairs(n - 1, nxt, res + nxt)

This runs much faster than your code and can go a lot higher before getting a stack overflow. But it will eventually get a stack overflow. We might do better adding memoization but ultimately, a simple iterative solution would be easier:

def climbStairs(self, n):  # doesn't use 'self', could be class method
    if 0 <= n <= 1:
        return 1

    a = b = 1

    for _ in range(n - 1):
        a, b = b, a + b

    return b

Answer 2

Your solution looks good but it's inefficient. You don't have to address if n==2 part. Keep it simple, like this:

class Solution:
    def climbStairs(self, n: int) -> int:
        if n == 0 or n == 1:
            return 1
        
        return self.climbStairs(n-1) + self.climbStairs(n-2)