I have this example df

col1 = [1,1,1,2,2,1,1,1,2,2,2]
col2 = [20, 23, 12, 44, 14, 42, 44, 1, 42, 62, 11]

data = {"col1": col1, "col2": col2}
df = pd.DataFrame(data)

I need to add a column that adds up the col2 every time the col1 is 1 and then the same for when it is 2. I have tried grouping by col1 but this skips every time there is a 2 in between
The expected output would be this.

col1    col2  col3
1       20    55 
1       23    55
1       12    55
2       44    58
2       14    58
1       42    87
1       44    87
1       1     87
2       42    115
2       62    115
2       11    115

Please let me know how to approach this