class Solution:
   def solve(self, mat):
      R, C = len(mat), len(mat[0])
      mass = {}
      id = 555
      def floodfill(r, c, id):
          nonlocal R, C, mat, mass
          if 0 <= r < R and 0 <= c < C and mat[r][c] == 1:
             mat[r][c] = id
             mass[id] += 1
             for r2, c2 in [(r + 1, c), (r − 1, c), (r, c + 1),
                  (r, c − 1)]:
                floodfill(r2, c2, id)
      for r in range(R):
          for c in range(C):
             if mat[r][c] == 1:
                id += 1
                mass[id] = 0
                floodfill(r, c, id)
      ans = max([val for val in mass.values()] + [1])
      for r in range(R):
          for c in range(C):
             if mat[r][c] != 0:
                continue
            island_set = set()
            for r2, c2 in [(r + 1, c), (r − 1, c), (r, c + 1),(r, c − 1)]:
                if 0 <= r2 < R and 0 <= c2 < C and mat[r2][c2]:
                   island_set.add(mat[r2][c2])
                ans = max(ans, 1 + sum([mass[island] for island in island_set]))
      return ans
ob = Solution()
matrix = [
   [1, 0, 1],
   [0, 0, 0],
   [1, 1, 0],
   [1, 1, 1]
]
print(ob.solve(matrix))

[
[1, 0, 1],
[0, 0, 0],
[1, 1, 0],
[1, 1, 1] ]

7