Python Set difference to find lost element from a duplicated array

Given two arrays which are duplicates of each other except one element, that is one element from one of the array is missing, we need to find that missing element.


Input:  A = [1, 4, 5, 7, 9]
        B = [4, 5, 7, 9]
Output: [1]
1 is missing from second array.

Input: A = [2, 3, 4, 5
       B = 2, 3, 4, 5, 6]
Output: [6]
6 is missing from first array.

We have existing solution for this problem please refer Find lost element from a duplicated array. We can solve this problem quickly in python using Set difference logic. Approach is very simple, simply convert both lists in Set and perform A-B operation where len(A)>len(B).





# Function to find lost element from a duplicate
# array
def lostElement(A,B):
     # convert lists into set
     A = set(A)
     B = set(B)
     # take difference of greater set with smaller
     if len(A) > len(B):
         print (list(A-B))
         print (list(B-A))
# Driver program
if __name__ == "__main__":
    A = [1, 4, 5, 7, 9]
    B = [4, 5, 7, 9]




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