Maximum sum of k connected elements of a matrix

Question

Given a grid with positive integer values and an integer K. What is the maximum sum of K connected elements ?

Here is a example of

Answer 1

I suppose you could meander around, memoizing as you go. I used mirror-image bitsets to represent the memoized paths so that they would be instantly recognizable from any direction they get constructed. Here's a version in Python (the hash length includes counts for paths from sizes one to six):

from sets import Set

def f(a,k):
  stack = []
  hash = Set([])
  best = (0,0) # sum, path
  n = len(a)

  for y in range(n):
    for x in range(n):
      stack.append((1 << (n * y + x),y,x,a[y][x],1))

  while len(stack) > 0:
    (path,y,x,s,l) = stack.pop()

    if l == k and path not in hash:
      hash.add(path)
      if s > best[0]:
        best = (s,path)
    elif path not in hash:
      hash.add(path)
      if y < n - 1:
        stack.append((path | (1 << (n * (y + 1) + x)),y + 1,x,s + a[y + 1][x],l + 1))
      if y > 0:
        stack.append((path | (1 << (n * (y - 1) + x)),y - 1,x,s + a[y - 1][x],l + 1))
      if x < n - 1:
        stack.append((path | (1 << (n * y + x + 1)),y,x + 1,s + a[y][x + 1],l + 1))
      if x > 0:
        stack.append((path | (1 << (n * y + x - 1)),y,x - 1,s + a[y][x - 1],l + 1))

  print best
  print len(hash)

Output:

arr = [[31,12,7,1,14]
      ,[23,98,3,87,1]
      ,[5,31,8,2,99]
      ,[12,3,42,17,88]
      ,[120,2,7,5,7]]

f(arr,6) 

""" 
(377, 549312) sum, path
1042 hash length
549312 = 00000
         01110
         11000
         10000 
"""

UPDATE: This question is similar to this one, Whats the fastest way to find biggest sum of M adjacent elements in a matrix, and I realized that a revision is needed in my suggestion to include formations extending from middle sections of the shapes. Here's my revised code, using sets to hash the shapes. It seems to me that a DFS ought to keep the stack size on the order of O(m) (although the search space is still huge).

from sets import Set

def f(a,m):
  stack = []
  hash = Set([])
  best = (0,[]) # sum, shape
  n = len(a)

  for y in range(n):
    for x in range(n):
      stack.append((a[y][x],Set([(y,x)]),1))

  while len(stack) > 0:
    s,shape,l = stack.pop()

    key = str(sorted(list(shape)))

    if l == m and key not in hash:
      hash.add(key)
      if s > best[0]:
        best = (s,shape)
    elif key not in hash:
      hash.add(key)
      for (y,x) in shape:
        if y < n - 1 and (y + 1,x) not in shape:
          copy = Set(shape)
          copy.add((y + 1,x))
          stack.append((s + a[y + 1][x],copy,l + 1))
        if y > 0 and (y - 1,x) not in shape:
          copy = Set(shape)
          copy.add((y - 1,x))
          stack.append((s + a[y - 1][x],copy,l + 1))
        if x < n - 1 and (y,x + 1) not in shape:
          copy = Set(shape)
          copy.add((y,x + 1))
          stack.append((s + a[y][x + 1],copy,l + 1))
        if x > 0 and (y,x - 1) not in shape:
          copy = Set(shape)
          copy.add((y,x - 1))
          stack.append((s + a[y][x - 1],copy,l + 1))

  print best
  print len(hash)

Output:

arr = [[31,12,7,1,14]
      ,[23,98,3,87,1]
      ,[5,31,8,2,99]
      ,[12,3,42,17,88]
      ,[120,2,7,5,7]]

f(arr,6)

"""
(377, Set([(1, 2), (1, 3), (1, 1), (2, 3), (3, 4), (2, 4)]))
2394 hash length
"""