Problem 846. Calculate the Hamming distance between two strings

Inspired by a similar Cody problem found here.

Parts of the problem description were taken from http://en.wikipedia.org/wiki/Hamming_distance.

The Hamming distance between two strings of equal length is the number of positions at which the corresponding symbols are different. In other words, it is the number of substitutions required to transform one string into another. Given two strings of equal length, compute the Hamming distance.

Example

If

s1 = 'cats'
s2 = 'dogs'

then

d = 3

because three substitutions are required to transform s1 into s2.

   cats => dats (substitute 'd' for 'c')
   dats => dots (substitute 'o' for 'a')
   dots => dogs (substitute 'g' for 't')