InterviewBit Problems   Stairs : Unsolved

About the Stairs : Unsolved category (1)
Can it be solved using unbounded knapsack(coin change) how? (3)
O(1) Solution : (3)
Easy C++ solution (with comments) (1)
SimpleO(n) DP Approach in CPP (1)
O(log n) approach (3)
Output for A = 0 is wrong (6)
Do anyone know the O(logn) approach mentioned in solution (3)
Easy DP solution in Java (Similar to fibonacci) (1)
For A = 0; output should be zero (2)
How will you come up with a logn solution? (3)
What is the its normal solution (2)
Runtime Error when submitting but works fine for custom input (1)
Getting index out of bounds on submitting but correct answer when testing (2)