Python | Find common elements in three sorted arrays by dictionary intersection

Given three arrays sorted in non-decreasing order, print all common elements in these arrays.

Examples:

Input:  ar1 = [1, 5, 10, 20, 40, 80]
        ar2 = [6, 7, 20, 80, 100]
        ar3 = [3, 4, 15, 20, 30, 70, 80, 120]
Output: [80, 20]

Input:  ar1 = [1, 5, 5]
        ar2 = [3, 4, 5, 5, 10]
        ar3 = [5, 5, 10, 20]
Output: [5, 5]



We have existing solution for this problem please refer Find common elements in three sorted arrays link. We can solve this problem quickly in python using intersection of dictionaries. Approach is simple,

  1. First convert all three lists into dictionaries having elements as keys and their frequencies as value, using Counter() method.
  2. Now perform intersection operation for three dictionaries, this will result us dictionary having common elements among three array list with their frequencies.
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# Function to find common elements in three
# sorted arrays
from collections import Counter
  
def commonElement(ar1,ar2,ar3):
     # first convert lists into dictionary
     ar1 = Counter(ar1)
     ar2 = Counter(ar2)
     ar3 = Counter(ar3)
     
     # perform intersection operation
     resultDict = dict(ar1.items() & ar2.items() & ar3.items())
     common = []
      
     # iterate through resultant dictionary
     # and collect common elements
     for (key,val) in resultDict.items():
          for i in range(0,val):
               common.append(key)
  
     print(common)
  
# Driver program
if __name__ == "__main__":
    ar1 = [1, 5, 10, 20, 40, 80]
    ar2 = [6, 7, 20, 80, 100]
    ar3 = [3, 4, 15, 20, 30, 70, 80, 120]
    commonElement(ar1,ar2,ar3)

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Output:

[80, 20]


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