my_queue.rb

class MyQueue
  def initialize(size)
    @size = 0
    @min_index = 0
    @queues = Array.new(size) { Array.new }
  end

  def push(cost, index)
    if cost < 0 || cost >= @queues.size
      raise "invalid cost: cost=#{cost}, size=#{@queues.size}"
    end
    @queues[cost] << [cost, index]
    @size += 1
  end

  def peek()
    i = 0
    while i < @queues.size
      q = @queues[i]
      return q[0] unless q.empty?
      i += 1
    end
    nil
  end

  def set_min_index(index)
    raise "Cannot set a smaller index than the current: index=#{index}, @min_index=#{@min_index}" if index < @min_index
    @min_index = index

    i = 0
    while i < @queues.size
      q = @queues[i]
      j = 0
      while j < q.size && q[j][1] < @min_index
        j += 1
      end
      q.shift(j)
      @size -= j
      break unless q.empty?
      i += 1
    end
  end
end

# q = MyQueue.new(11)
# q.push(0, 0)
# q.push(8, 1)
# q.push(0, 2)
# q.push(8, 3)
# q.push(10, 4)
# p q.peek
# q.set_min_index(1)
# p q.peek
# q.set_min_index(2)
# p q.peek
# q.set_min_index(3)
# p q.peek
# q.set_min_index(4)
# p q.peek

my_queue_2.rb

class MyQueue
  def initialize(size)
    @size = 0
    @min_index = 0
    @min_value = nil
    @min_value_queue = 0
    @queues = Array.new(size) { Array.new }
  end

  def push(cost, index)
    if cost < 0 || cost >= @queues.size
      raise "invalid cost: cost=#{cost}, size=#{@queues.size}"
    end
    @queues[cost] << [cost, index]
    @size += 1
    if @min_value.nil? || @min_value[0] > cost
      @min_value = @queues[cost].first
      @min_value_queue = cost
    end
  end

  def peek()
    @min_value
  end

  def set_min_index(index)
    raise "Cannot set a smaller index than the current: index=#{index}, @min_index=#{@min_index}" if index < @min_index
    @min_index = index

    i = @min_value_queue
    while i < @queues.size
      q = @queues[i]
      j = 0
      while j < q.size && q[j][1] < @min_index
        j += 1
      end
      q.shift(j)
      @size -= j
      unless q.empty?
        @min_value = q.first
        @min_value_queue = i
        return
      end
      i += 1
    end
    @min_value = nil
    @min_value_queue = 0
  end
end

# q = MyQueue.new(11)
# q.push(0, 0)
# q.push(8, 1)
# q.push(0, 2)
# q.push(8, 3)
# q.push(10, 4)
# p q.peek
# q.set_min_index(1)
# p q.peek
# q.set_min_index(2)
# p q.peek
# q.set_min_index(3)
# p q.peek
# q.set_min_index(4)
# p q.peek