70. Climbing Stairs

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