The idea is that as Fibonnaci values are computed you store them in the array A. So when another F(n) is to be computed either A[n] is still 0 or else A[n] is already equal to F(n).

If A[n] is 0, you have to make the usual recursive calls to compute F(n). Then store this value in A[n] AND return the value.

If A[n] is not 0, that means you have already computed F(n), so just return A[n].