Group the Numbers With the Same Sum
Pyth, 122 - 20 - 15 = 87
=Z/lzQ=ks^lz.5Jm]dUzL[-bk+bk?tb%bkb?hb%hbkb)FNJIgNZB~Jm+NksmybN;|jbS{msm+@zk@S*Z&lt;GQxsdkUzfqSsTUz^fqsmv@*ZzbY/smvdzQJQ&quot;None

Changes:

130 -&gt; 120: Switched to newline separated input.

120 -&gt; 134: Fixed a bug involving groups not of size equal to the side length of the matrix.

134 -&gt; 120: Prints all solutions, including ones equivalent under group renaming.

120 -&gt; 122: Fixed a bug where only paths would be generated, instead of all legal groups.


Test run:
pyth programs/sum_group.pyth &lt;&lt;&lt; '156790809
3'
1a5a6b7c9a0b8c0c9b
1a5a6c7b9a0c8b0b9c
1b5b6a7c9b0a8c0c9a
1b5b6c7a9b0c8a0a9c
1c5c6a7b9c0a8b0b9a
1c5c6b7a9c0b8a0a9b


pyth programs/sum_group.pyth &lt;&lt;&lt; '156790808
3'
None

pyth programs/sum_group.pyth &lt;&lt;&lt; '1111     
2'
1a1a1b1b
1a1b1a1b
1b1a1b1a
1b1b1a1a

Explanation:
Pyth code           (Pseudo)-Python code              Comments

(implicit)          z = input()                       z is the digit string
(implicit)          Q = eval(input())                 S is the number of groups
(implicit)          G = 'abcdefghijklmnopqrstuvwxyz'
=Z/lzQ              Z = len(z)/Q                      Z is the size of each group.
=ks^lz.5            k = int(len(z) ** .5)             k is the side length of the matrix.
Jm]dUz              J = map(lambda d:[d], range(len(z))) Locations are encoded as numbers.
L                   def y(b): return                  y will be the transition function.
 [-bQ                         [b-k,                   Move up - the row above is k less.
  +bQ                          b+k,                   Move down - the row below is k more.
  ?tb%bkb                      b-1 if b%k else b      Move left, unless at the left edge.
  ?hb%hbkb)                    b+1 if (b+1)%k else b] Move right, unless at right edge.
FNJ                 for N in J:                       This constructs the list of all
   IgNZB                       if N[Z-1]: break       Z-length connected groups.
   ~Jm+Nk                      J+=map(lambda k: N+[k],  Append to J the group of N +
      smybN                          sum(map(lambda b:  anything reachable from
                                     y(b),N)))        anywhere in N.
   ;                (end for)
|                   or                                Print first truthy thing between
 S{                 sorted(set(                       Unique elements in sorted order of
   ms               map(lambda b:sum(                 Map+sum over allowable combinations
     m+@zd          map(lambda d:z[d]+                Character in original digit string
       @S*Z&lt;GQ      sorted(G[:Q]*Z)[                  Repeated and sorted early alphabet
        xsbd        sum(b).index(d)],                 At index of number in sum of groups
      Uz                range(len(z)))                Over possible indexes.
   f                filter(lambda T:                  To generate allowable combinations, 
                                                      we will filter all groups of Q paths.
    qSsTUz          sorted(sum(T)) == range(len(z))   Ensure all locations are visited.
    ^                                                 Combinations of
     f              filter(lambda Y:                  Filter over connected Z-length groups
      qsm           equal(sum(map(lambda k:           Sum of the values of the group
         v@*ZzkY    eval((z*Z)[k]),Y)                 In the original digit string
       /smvbzQ      sum(map(lambda b:eval(b),z))/Q    must equal the sum of all values in z
                                                      divided by the number of groups.
      J             J                                 Filter over connected Z-length groups
     Q              Q                                 Combinations of length Q
 &quot;None              &quot;None&quot;                            If the above was empty, print &quot;None&quot;