Pandas dataframe assign value to lists based on other columns

Question

I have a dataframe like this:

   id  cola colb
0   0   0    [1]
1   1   1    [2,3,4]
2   1   2    [2,3,4]
3   2   3    [3]
4   2   4    [3]
5   2   5    [4]
6   3   6    [5]
...

and want it to convert to:

   id  cola colb      colc
0   0   0    [1]       
1   1   1    [2,3,4]   0
2   1   2    [2,3,4]   0
3   2   3    [3]       1,2
4   2   4    [3]       1,2
5   2   5    [4]       1,2
6   3   6    [5]       1,2,3,4
...

The rule is:

Take first row for example, colb=[1], corresponding cola=0, then cola=0 is appended to cell list in colc for id=[1].

Take second row for example, colb=[2,3,4], corresponding cola=1, then cola=1 is appended to cell list in colc for id=[2,3,4].

Take third row for example, colb=[2,3,4], corresponding cola=2, then cola=2 is appended to cell list in colc for id=[2,3,4]. ...

Here is what I have tried so far:

df['colc'] = [[] for _ in range(len(df))]
for i in range(len(df['colb'])):
    lista = df.colb[i]
    for j in lista:
        df.colc.loc[df.id==j].append(pd.Series(df.cola[i])) 

But it seems that something is wrong at df.colc.loc[df.id==j], no value has appended to the list. Is there anyway to do this? Could someone be so kind to help me? Thanks!