Problem Statement  
A TriFibonacci sequence begins by defining the first three elements A[0], A[1] and A[2]. The remaining elements are calculated using the following recurrence:A[i] = A[i1] + A[i2] + A[i3] You are given a int[] A which contains exactly one element that is equal to 1, you must replace this element with a positive number in a way that the sequence becomes a TriFibonacci sequence. Return this number. If no such positive number exists, return 1.  
Definition  
 
Notes  
  The constraints for the elements of the input int[] A do not necessarily apply for the replacement to the missing element.  
Constraints  
  A will contain between 4 and 20 elements, inclusive.  
  Each element of A will be 1 or between 1 and 1000000, inclusive.  
  Exactly one element of A will be 1.  
Examples  
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