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Common Elements in Two Lists

Certification: Basic Level Accuracy: 71.79% Submissions: 39 Points: 10

Write a Python program that finds common elements in two lists, considering duplicates.

Example 1
    

  • Input: list1 = [1, 2, 2, 3], list2 = [2, 2, 3, 4]
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  • Output: [2, 2, 3]
    • 

  • Explanation: 
      

    • Step 1: Create a dictionary to count occurrences of each element in the first list.
      • 

    • Step 2: Create an empty result list to store common elements.
      • 

    • Step 3: Iterate through the second list, checking if each element exists in the dictionary.
      • 

    • Step 4: If the element exists and has a count > 0, add it to the result and decrement its count.
      • 

    • Step 5: Return the common elements with correct multiplicity: [2, 2, 3].
      • 

    •
Example 2
    

  • Input: list1 = [1, 2, 3], list2 = [4, 5, 6]
    • 

  • Output: []
    • 

  • Explanation: 
      

    • Step 1: Create a dictionary to count occurrences of each element in the first list.
      • 

    • Step 2: Create an empty result list to store common elements.
      • 

    • Step 3: Iterate through the second list, checking if each element exists in the dictionary.
      • 

    • Step 4: No common elements are found between the two lists.
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    • Step 5: Return an empty list [].
      • 

    •
Constraints
    

  • 1 ≤ len(list1), len(list2) ≤ 10^3
    • 

  • -10^5 ≤ list1[i], list2[i] ≤ 10^5
    • 

  • Time Complexity: O(n) where n is the total number of elements in both lists
    • 

  • Space Complexity: O(n) to store the frequency counter dictionary
    •
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Hint
Solution

Solution Hints

    
  
  • Use Counter to count occurrences in both lists.
    • 
  
  • Iterate through one list and check common counts in the other.
    •

Steps to solve by this approach:

 Step 1: Use Counter to count occurrences of each element in both lists.

 Step 2: Initialize an empty list to store common elements.

 Step 3: Iterate through the elements in the first count dictionary.

 Step 4: Check if the element also exists in the second count dictionary.

 Step 5: If it exists in both, add it to the result list the minimum number of times it appears in either list.
Step 6: Return the list of common elements including duplicates.

Submitted Code :