Leetcode 0070. 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　

Easy

Description

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

• There are two ways to climb to the top.

  1. 1 step + 1 step
  2. 2 steps

Example 2:

Input: n = 3
Output: 3

• 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 {
    /**
     * Iterative Fibonacci Sequence
     * Time Complexity: BigO(n)
     * Space Complexity: BigO(1)
     */
    public int climbStairs(int n) {
        if(n == 1)
            return 1;
        if(n == 2)
            return 2;

        int prev = 1;
        int cur = 2;
        for(int i = 3; i <= n; i++) {
            int next = cur + prev;
            prev = cur;
            cur = next;
        }
        return cur;
    }
}