Copied to Clipboard
Count ways to reach the n'th stair
Medium Accuracy: 42.67% Submissions: 47607 Points: 4

There are n stairs, a person standing at the bottom wants to reach the top. The person can climb either 1 stair or 2 stairs at a time. Count the number of ways, the person can reach the top (order does matter).

Example 1:

n = 4
Output: 5
You can reach 4th stair in 5 ways. 
Way 1: Climb 2 stairs at a time. 
Way 2: Climb 1 stair at a time.
Way 3: Climb 2 stairs, then 1 stair
and then 1 stair.
Way 4: Climb 1 stair, then 2 stairs
then 1 stair.
Way 5: Climb 1 stair, then 1 stair and
then 2 stairs.

Example 2:

n = 10
Output: 89 
There are 89 ways to reach the 10th stair.

Your Task:
Complete the function countWays() which takes the top stair number m as input parameters and returns the answer % 10^9+7.

Expected Time Complexity : O(n)
Expected Auxiliary Space: O(1)

1 ≤ n ≤ 104

We are replacing the old Disqus forum with the new Discussions section given below.
Click here to view old Disqus comments.

to report an issue on this page.


We strongly recommend solving this problem on your own before viewing its editorial. Do you still want to view the editorial?


All Submissions

My Submissions:

Login to access your submissions.

Count ways to reach the n'th stair

Output Window