LeetCode - Climbing Stairs

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Note: Given n will be a positive integer.

Example 1:

Input: 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

Example 2:

Input: 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

Java Solution :

class Solution {
    public int climbStairs(int n) {
        if(n == 0 || n == 1 || n == 2)
            return n;
        int previousstep = 2;
        int prepreviousstep = 1;
        for(int i = 3; i <= n ; i++){
            int current = previousstep + prepreviousstep;
            prepreviousstep = previousstep;
            previousstep = current;
        }
        return previousstep;
    }
}

Time Complexity : O(n)

Space Complexity : O(1)

Approach : Dynamic Programming Tabularization.