# Climbing Stairs Solution 2

By: Jay the Code Monkey
Posted: Jul 1, 2021 Updated: Apr 18, 2023

Climbing Stairs

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# Problem 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?

# Examples

Ex 1) Input: n = 2 Output: 2

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

Ex 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

# Thought Process

  • We previously came up with a solution for this problem by using a recursive formula that represents how many distinct ways we can climb the stairs.

  • The issue with the previous solution is it has a time complexity of O(2n) which is very inefficient.

  • To come up with a better solution we'll be drawing out multiple recursion trees to help us see a pattern.

  • When n = 1 we have:

  • From the diagram we can see there is 1 distinct way to climb to the top when n = 1 since we can ignore the scenarios where we take extra steps.

  • When n = 2 we have:

  • So, we have 2 distinct ways to climb to the top.

  • When n = 3 we have:

  • So, we have 3 distinct ways to climb to the top.

  • When n = 4 we have:

  • So, we have 5 distinct ways to climb to the top.

  • Now, let's take a look at our cases from n = 1 to n = 2 and see if we can find a pattern:

  • Here, waysToClimb denotes our function for determining how many unique ways we can climb the stairs.

  • This sequence of numbers follows the Fibonacci sequence with the only difference being the value of the 1st and 2nd terms.

  • Here, the 1st term has a value of 1 and the 2nd term has a value of 2.

  • Also, notice that we know we have two ways to climb the stairs to reach the nth step.

  • If we take 1 step, then we're n - 1 steps closer to n, and if we take 2 steps then we're n - 2 steps closer to n.

  • So, we can get the distinct number of ways to climb the steps by summing up the ways of climbing to the n - 1 step and the ways of climbing to the n - 2 step using the following formula:

  • The Fibonacci sequence is the following series of numbers:

  • The next number is found by adding up the two numbers before it.

  • For example, 5 is found by adding 2 and 3.

  • The first two terms are called seed numbers, which we'll denote as:

  • Here's the formula used to characterize the sequence:

  • The seed numbers for our problem are:

  • Now that we know our pattern of climbing the stairs follows a Fibonnaci sequence we can implement the sequence in our code using the waysToClimb formula and the seed numbers we came up with.

  • This will improve the time complexity of our solution from the O(2n) solution we previously came up with.

# Implementation

var climbStairs = function(n) {
  if (n === 1) {
    return 1;
  } else if (n === 2) {
    return 2;
  } else {
    let waysToClimb1 = 1;
    let waysToClimb2 = 2;
    let waysToClimb;
    let i = 1;

    while (n - 1 > i) {
      waysToClimb = waysToClimb2 + waysToClimb1;
      waysToClimb1 = waysToClimb2;
      waysToClimb2 = waysToClimb;

    return waysToClimb;

let n = 4;

# Analysis

  • Since we're looping over the length of n - 1 our time complexity is O(n) which is a much more efficient solution!

  • See if you can come up with an even more efficient solution using more properties of the Fibonnaci sequence!

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