I want to turn `lst = ['123', '456', '789']` into `lst = [[1,2,3], [4,5,6], [7,8,9]]`.

I tried:

```
for i in range(len(lst)):
    lst[i] = list(lst[i])

```

This produced `lst = [['1','2','3'], ['4','5','6'], ['7','8','9']]`.

How can I turn those string numbers into integers?

Note: I can't use `map`.