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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:

Input:
n = 4
Output: 5
Explanation:
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:

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


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)

Constraints:
1 ≤ n ≤ 104

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