Question: This morning I started Google's foo.bar challenge. The quest was comparing 2 lists and returning non-same values.

> 
> For example, given the lists x = [13, 5, 6, 2, 5] and y = [5, 2, 5,
> 13], the function solution(x, y) would return 6 because the list x
> contains the integer 6 and the list y doesn't.
> 
> 
> 

So I wrote this code;

```
x = [13, 5, 6, 2, 5]
y = [5, 2, 5, 13]

list_difference1 = [item for item in x if item not in y]
list_difference2 = [item for item in y if item not in x]

list_difference = list_difference2 + list_difference1
list_difference

```

output: [6]

This is what it looks like in the script;

```
def solution(x, y):
    list_difference1 = [item for item in x if item not in y]
    list_difference2 = [item for item in y if item not in x]

    list_difference = list_difference2 + list_difference1

    return list_difference

```

I can't understand what I have done wrong.

Answer: 
You're supposed to return `6`, not `[6]`.
Use [sets](https://docs.python.org/3/tutorial/datastructures.html#sets), which are faster than lists:

```
x = [13, 5, 6, 2, 5]
y = [5, 2, 5, 13]

def solution(x, y):
    return list(set(x) ^ set (y))

print(solution(x, y))
# [6]

```

Otherwise, you code seems correct (just not as efficient):

```
def solution2(x, y):
    list_difference1 = [item for item in x if item not in y]
    list_difference2 = [item for item in y if item not in x]
    list_difference = list_difference2 + list_difference1
    return list_difference

print(solution2(x, y))
# [6]

```