70. Climbing Stairs

Easy

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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

class Solution:
    def climbStairs(self, n: int) -> int:
        d = {}
        def recursion(n, d):
            if n == 1:
                return 1
            elif n == 2:
                return 2
            elif n in d:
                return d[n]
            sum = recursion(n-1, d) + recursion(n-2, d)
            d[n] = sum
            return sum
        return recursion(n, d)