In terms of lowest computational complexity, how to create a new pandas column which increments by a certain number based on another column

Question

I have this column of numbers, sorted by value

import pandas as pd 
  
# initialize list of lists 
data = [1, 1, 1, 1, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 5, 6, 6, 6]