0070_Climbing_Stairs.md

70. Climbing Stairs

Easy

You are climbing a staircase. It takes n steps to reach the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Example 1:

Input: n = 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

Example 2:

Input: n = 3
Output: 3
Explanation: There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step

Constraints:

1 <= n <= 45

Solution

class Solution:
    def climbStairs(self, n: int) -> int:
        dp = {}
        
        def helper(steps_left):
            count = 0
            
            if steps_left == 0:
                return 1
            
            if dp.get(steps_left) is not None:
                return dp.get(steps_left)
            
            if steps_left - 1 >= 0:
                count += helper(steps_left - 1)
            
            if steps_left - 2 >= 0:
                count += helper(steps_left - 2)
            
            dp[steps_left] = count
            return dp[steps_left]
                
        
        return helper(n)