Full question:
Given N step stair, how many number of ways can you climb if you use either 1 or 2 at a time?

Answer

It will be F(n+1) fibonacci number, there Fibonacci numbers are defined by the recurrence relation:

F(n) = F(n-2) + F(n-1)

for N stair number of ways will be

N=1 => 1 = F(2)
N=2 => 2 = F(3)
N=3 => 3 = F(4)
N=4 => 5 = F(5)
N=5 => 8 = F(6)
N=6 => 13 = F(7)
....

N=k => F(k+1)




Fibonacci numbers on Wikipedia








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