1. 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

Approach

• It is a Fibonacci sequence
• Maintain one variable to hold next sum and one to store last value and return sum one
• Time: O(n) Space: O(1)

class Solution {
    public int climbStairs(int n) {
        int a = 1, b = 1;
        for (int i = 0; i < n - 1; i++) { //works with i < n as well
            int t = a;
            a += b;
            b = t; 
        }
        return a;
    }
}