The 0-1 Knapsack problem - Regression equation

Question

I'm trying to figure out the regression equation for the 0-1 Knapsack problem recursive algorithm:

// Returns the maximum value that can be put in a knapsack of capacity W
int knapSack(int W, int wt[], int val[], int n)
{
   // Base Case
   if (n == 0 || W == 0)
       return 0;

   // If weight of the nth item is more than Knapsack capacity W, then
   // this item cannot be included in the optimal solution
   if (wt[n-1] > W)
       return knapSack(W, wt, val, n-1);

   // Return the maximum of two cases: 
   // (1) nth item included 
   // (2) not included
   else return max( val[n-1] + knapSack(W-wt[n-1], wt, val, n-1),
                    knapSack(W, wt, val, n-1)
                  );
}

What I have is only the answer but I don't really know how to get to it:

T(n) = 2T(n) + O(1) = O(2^n)

Any ideas?