LeetCode-in-All

70. Climbing Stairs

Easy

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:

Solution

defmodule Solution do
  @spec climb_stairs(n :: integer) :: integer
  def climb_stairs(1), do: 1
  def climb_stairs(2), do: 2
  def climb_stairs(n), do: do_climb_stairs(1, 2, n - 2)
  
  defp do_climb_stairs(a, b, n) when n > 0, do: do_climb_stairs(b, a + b, n - 1)
  defp do_climb_stairs(_, b, _), do: b
end