Mistake in Python code

There is an array of integers. There are also disjoint sets, A and B, each containing integers. You like all the integers in set A and dislike all the integers in set B. Your initial happiness is 0. For each integer in the array, if i in A, you add 1 to your happiness. If i in B, you add -1 to your happiness. Otherwise, your happiness does not change. Output your final happiness at the end.

Input Format

The first line contains integers n and m separated by a space. The second line contains n integers, the elements of the array. The third and fourth lines contain m integers, A and B respectively.

Output Format

Output a single integer, your total happiness.

Sample Input

3 2

1 5 3

3 1

5 7

Sample Output

Can someone please explain what is wrong with this solution? It passes some test, but fails on others.

input()
array = set(input().split())
set1 = set(input().split())
set2 = set(input().split())
res = len(set1 & array) - len(set2 & array)
print(res)

Solution

The problem is that you're transforming your inputs to sets, which in turn removes the duplicates. If you have repeated values in your input, with the set you're only adding/substracting 1 to the resulting happiness. If that's correct, your code is fine. If not, then you should work with lists rather than sets.

The code could be something like this:

# The first part should stay the same, without the set() call on array
input()
array = input().split()
list1 = set(input().split())
list2 = set(input().split())
# Now we use some list comprehension to get the happiness result
res = sum([1 for elem in array if elem in list1]) - sum([1 for elem in array if elem in list2])

The first sum accumulates the positive points, and the second one the negatives. It works with multiple occurences, adding/substracting one point per each.

EDIT

A more clear approach, to understand the for loop

# The first part should stay the same, without the set() call on array
input()
array = input().split()
list1 = set(input().split())
list2 = set(input().split())
# We create a variable res which will store the resulting happiness, initially 0
res = 0
# Now we iterate through the elements in array and check wheter they should add or substract
for elem in array:
    # If the element is in list1, we add 1 to res
    if elem in list1:
        res += 1
    # If the element is in list2, we substract 1 from res
    elif elem in list2:
        res -= 1