I have two list of list and I want to check what element of l1 is not in l2 and add all this element in a new list ls in l2. For example,

input:   

l1 = [[1,2,3],[5,6],[7,8,9,10]]  

l2 = [[1,8,10],[3,9],[5,6]]  

output:  

l2 = [[1,8,10],[3,9],[5,6],[2,7]]  

I write this code but it is seem doesn't work...

```
ls = []
for i in range(len(l1)):
  for j in range(len(l1[i])):
    if l1[i][j] not in l2 and l1[i][j] not in ls:
      ls.append(l1[i][j])
l2.append(ls)

```

I code this in python. Any ideas where can be the problem ?