70. Climbing Stairs

Fibonaci sequence

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?

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

Constraints:

• 1 <= n <= 45

# @param {Integer} n
# @return {Integer}
def climb_stairs(n)
  return 1 if n == 1
  return 2 if n == 2
  fb = []
  fb[1] = 1
  fb[2] = 2
  3.upto(n) do |x|
    fb[x] = fb[x-1] + fb[x-2]
  end
  fb.last
end