Python – Inverse Dictionary Values List

Given a Dictionary as value lists, inverse it, i.e map elements in list to keys and create new values list.

Input : test_dict = {1: [2, 3], 2: [3], 3: [1]} Output : {2: [1], 3: [1, 2], 1: [3]} Explanation : List elements mapped with their keys. Input : test_dict = {1: [2, 3, 4]} Output : {2: [1], 3: [1], 4: [1]} Explanation : List elements mapped with their keys.

Method : Using defaultdict() + loop

This is a way in which this task can be performed. In this, we initialize result keys with dictionary list, and iterate using loop to assign each value its keys, and reform the result dictionary values list.

Python3

# Python3 code to demonstrate working of
# Inverse Dictionary Values List
# Using
from collections import defaultdict

# initializing dictionary
test_dict = {1: [2, 3], 2: [3], 3: [1], 4: [2, 1]}

# printing original dictionary
print("The original dictionary is : " + str(test_dict))

# initializing empty list as Values
res = defaultdict(list)

# using loop to perform reverse mapping
for keys, vals in test_dict.items():
   for val in vals:
       res[val].append(keys)

# printing result
print("The required result : " + str(dict(res)))

Output

The original dictionary is : {1: [2, 3], 2: [3], 3: [1], 4: [2, 1]}
The required result : {2: [1, 4], 3: [1, 2], 1: [3, 4]}

Time Complexity: O(n*n) where n is the number of elements in the dictionary
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the dictionary