Sum Unique Elements

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Unit 3 Session 2 (Click for link to problem statements)

U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

• What if lst1 is empty?
  • In this case, the total sum of elements is 0.
• What if lst2 is empty?
  • In this case, you don't need to worry about lst1 elements appearing in lst2. But you still need to check if elements occur more than once in lst1!

P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Create a frequency map of elements across both lists, then add up any element in lst1 with a frequency of 1.

1) Create a frequency map¹ for all elements across both lists
2) For each element in lst1, if the frequency is 1, add to sum
3) Return the sum

¹ see Frequency Count for psuedocode

⚠️ Common Mistakes

• Two separate frequency maps will slow down your solution -- how can you do it with just one?

I-mplement

def sum_of_unique_elements(lst1, lst2):
    count = {}
    for num in lst1 + lst2:
        if num in count:
            count[num] += 1
        else:
            count[num] = 1
    sum_unique = 0
    for num in lst1:
        if count[num] == 1:
            sum_unique += num
    return sum_unique