Problem Statement  
The distance between two strings with respect to a letter is defined as (n_{1}  n_{2})^{2}, where n_{1} and n_{2} are the number of occurrences (both lowercase and uppercase) of that letter in the first and second strings, respectively. The distance between two strings with respect to a third string is the sum of the distances between the two strings with respect to each letter in the third string. You will be given three Strings a, b and letterSet. All the letters in letterSet will be distinct. Return the distance between a and b with respect to letterSet.  
Definition  
 
Constraints  
  a will contain between 1 and 50 characters, inclusive.  
  a will contain only letters ('a''z', 'A''Z').  
  b will contain between 1 and 50 characters, inclusive.  
  b will contain only letters ('a''z', 'A''Z').  
  letterSet will contain between 1 and 26 characters, inclusive.  
  letterSet will contain only lowercase letters ('a''z').  
  Each character in letterSet will be distinct.  
Examples  
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