Problem Statement  
Given a sequence of K elements, we can calculate its difference sequence by taking the difference between each pair of adjacent elements. For instance, the difference sequence of {5,6,3,9,1} is {65,36,93,19} = {1,3,6,10}. Formally, the difference sequence of the sequence a_{1}, a_{2}, ... , a_{k} is b_{1}, b_{2}, ... , b_{k1}, where b_{i} = a_{i+1}  a_{i}.
The derivative sequence of order N of a sequence A is the result of iteratively applying the above process N times. For example, if A = {5,6,3,9,1}, the derivative sequence of order 2 is: {5,6,3,9,1} > {1,3,6,10} > {31,6(3),106} = {4,9,16}.
You will be given a sequence a as a int[] and the order n. Return a int[] representing the derivative sequence of order n of a.
  Definition   Class:  DerivativeSequence  Method:  derSeq  Parameters:  int[], int  Returns:  int[]  Method signature:  int[] derSeq(int[] a, int n)  (be sure your method is public) 
    Notes    The derivative sequence of order 0 is the original sequence. See example 4 for further clarification.   Constraints    a will contain between 1 and 20 elements, inclusive.    Each element of a will be between 100 and 100, inclusive.    n will be between 0 and K1, inclusive, where K is the number of elements in a.   Examples  0)     Returns: {1, 3, 6, 10 }  The first example given in the problem statement. 

 1)     Returns: {4, 9, 16 }  The second example given in the problem statement. 

 2)     3)     Returns: {0, 0, 0, 0, 0 }  After 1 step, they all become 0. 

 4)    
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