Problem Statement: Write a function to find the missing element from given two arrays.
Difficulty level: Easy
Test Cases:
 [1, 2, 3, 4, 5, 6, 7],[3, 7, 2, 1, 4, 6] > 5
 [5, 7, 7, 5, 7], [7, 7 ,5, 5] > 7
 [9, 8, 7, 6, 5, 4, 3, 2, 1],[9, 8, 7, 5, 4, 3, 2, 1] > 6
 [1, 2, 4, 7, 9], [7, 9, 4, 2] > 1
There are number of ways this problem can be solved, below are 4 possible solutions
Algorithm

Dictionary Method
 Initialise a dictionary.
 Iterate through the first array,
 If element at current iteration exists in dictionary,
 Increment it's value by 1.
 Else,
 Add the element as key and value as 1 in dictionary
 Iterate through the second array,
 If element at current iteration exists in dictionary,
 Decrement it's value by 1.
 Else,
 Add the element as key and value as 1 in dictionary.
 for every element in dictionary
 if value of the element is not 0
 return the element
 if value of the element is not 0

Exclusive OR (XOR) Method
 Initialise variable result to 0
 Combine both the arrays.
 Iterate through the combined array
 Do XOR operation of every element with result
 Return the result, which will be the missing element

Sort Method
 Sort both the given arrays.
 Iterate over both the arrays simultaneously.
 At a given iteration if value of both the iterators are different,
 return the element from first array, as it is the missing element.

Sum Method
 Calculate the sum of all the elements in both the arrays.
 Subtract the sum of second array from the first array.
 The result will be the missing element.
Time and Space complexity
 Dictionary Method
 Time Complexity: O(n)
 Space Complexity: O(n)
 XOR Method
 Time Complexity: O(n)
 Space Complexity: O(n)
 Sort Method
 Time Complexity: O(nlogn)
 Space Complexity: O(n)
 Sum Method
 Time Complexity: O(n)
 Space Complexity: O(1)
Code
class MissingElement(object):
def missingElement(self, arr1, arr2):
count = {}
for element in arr1:
if element in count:
count[element] += 1
else:
count[element] = 1
for element in arr2:
if element in count:
count[element] = 1
else:
count[element] = 1
for k in count:
if count[k] != 0:
return k
def missingElement2(self, arr1, arr2):
result = 0
for element in arr1 + arr2:
result ^= element
return result
def missingElement3(self, arr1, arr2):
arr1.sort()
arr2.sort()
for ele1, ele2 in zip(arr1, arr2):
if ele1 != ele2:
return ele1
return arr1[1]
def missingElement4(self, arr1, arr2):
return abs(sum(arr1)  sum(arr2))
Unit Test
import unittest
from missingElement import MissingElement
class TestMisssingElement(unittest.TestCase):
def test_ele(self, sol):
self.assertEqual(sol([5, 5, 7, 7], [5, 7, 7]), 5)
self.assertEqual(sol([1, 2, 3, 4, 5, 6, 7],
[3, 7, 2, 1, 4, 6]), 5)
self.assertEqual(sol([9, 8, 7, 6, 5, 4, 3, 2, 1],
[9, 8, 7, 5, 4, 3, 2, 1]), 6)
print("All test cases passed")
def main():
test = TestMisssingElement()
element = MissingElement()
test.test_ele(element.missingElement)
test.test_ele(element.missingElement2)
test.test_ele(element.missingElement3)
test.test_ele(element.missingElement4)
if __name__ == "__main__":
main()
Discussion (2)
Excellent! I was actually looking for this kind of program. Thanks for sharing :)
Java #Python #Challenge
Glad you liked it. 🙂