"""
70. Climbing Stairs
Difficulty: Easy
https://leetcode.com/problems/climbing-stairs/

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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:
        if n == 1:
            return 1
        if n == 2:
            return 2

        first, second = 1, 2

        for _ in range(3, n + 1):
            third = first + second
            first, second = second, third

        return second