70 Climbing Stairs

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

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class Solution:
    def climbStairs(self, n: int) -> int:
        if n < 3:
            return n
        prev_2 = 1
        prev_1 = 2
        for i in range(2, n):
            prev_1, prev_2 = prev_2 + prev_1, prev_1
        return prev_1