The Number of Island. http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=0067

number_of_island.rb

class NumberOfIsland
  (class << self; self; end;).class_eval do
    def output(input)
      input.split("\n\n").inject([]) { |result, line| result << run(line) }.join("\n")
    end

    def run(input, x = 0, y = 0, checked = [], count = 0)
      ary = convert_to_array(input)
      ary.size.times do
        row = []
        ary[0].size.times do
          row << nil
        end
        checked << row
      end

      count(ary, checked)
    end

    def count(ary, checked, x = 0, y = 0, count = 0)
      return count if ary.size <= x

      if ary[x][y] == 1 && checked[x][y] == nil
        count += 1
        checked = check(ary, checked, x, y) if ary[x][y] == 1
      end

      if ary[x].size >= y + 1
        return count(ary, checked, x, y + 1, count)
      else
        return count(ary, checked, x + 1, 0, count)
      end
    end

    def check(ary, checked, x, y)
      return checked if checked[x][y] == true
      return checked if ary[x][y] == 0
      checked[x][y] = true if ary[x][y] == 1

      checked = check(ary, checked, x + 1, y) if ary.size > x + 1
      checked = check(ary, checked, x, y + 1) if ary[x].size > y + 1
      checked = check(ary, checked, x - 1, y) if 0 <= x - 1
      checked = check(ary, checked, x, y - 1) if 0 <= y - 1
      return checked
    end

    def convert_to_array(input)
      input.each_line.inject([]) { |res, line| res << line.chomp.split('').map { |n| n.to_i } }
    end
  end
end

input = <<"EOS"
111100001111
111000001111
110000001111
100000001111
000100010000
000000111000
000001111100
100011111110
110001111100
111000111000
111100010000
000000000000

010001111100
110010000010
010010000001
010000000001
010000000110
010000111000
010000000100
010000000010
010000000001
010010000001
010010000010
111001111100

000000000000
111111111111
100010100001
100010100001
100010100001
100010100001
100100100101
101000011101
100000000001
100000000001
111111111111
100000000001
EOS

if __FILE__ == $0
  p NumberOfIsland.output(input)
end