Most Common Element Across Categories

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Your question is Most Common Element Across Categories. Start with the requirements on the right.

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Problem

Facebook surfaces such as Facebook Reels and Feed may group content labels into separate categories. Given a mapping from category names to lists of item names, return the item that appears most often across all categories.

Count every occurrence, including repeated appearances within the same category. If multiple items have the same highest frequency, return the lexicographically smallest item. At least one item appears across all category lists.

Formal Specification

Implement most_common_item(categories), where categories is a dictionary mapping strings to lists of strings. Return a single string containing the most frequent item.

Constraints

1 <= len(categories) <= 10^4
Each category list contains between 0 and 10^4 items
The total number of items across all category lists is at most 10^5
Item names are non-empty strings
The combined category lists contain at least one item

Function Signature

def most_common_item(categories):

Your solutionPython 3

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