Retrieving the Unique Number from a Dictionary

In the problem of finding a single number in an array where every other number appears twice, we use a dictionary to keep track of the frequency of each number. After processing all numbers, the dictionary will contain only one key—the number that appears exactly once. To retrieve this number, we use the expression list(unique.keys())[0]. Here’s a step-by-step breakdown of what this expression does:

1. unique.keys()

• What It Does:This method returns a view object of the dictionary’s keys.In Python, a dictionary view object is a dynamic view on the dictionary’s keys. It reflects changes to the dictionary in real time.

2. list(unique.keys())

• What It Does:Converts the view object returned by unique.keys() into a list.
• Lists are indexable, which allows us to access specific elements by their position.

3. list(unique.keys())[0]

• What It Does:Accesses the first element of the list created from the dictionary’s keys.
• Since we expect only one key to remain in the dictionary (the number that appears only once), this will give us that single number.

Summary

After processing the array of numbers, the dictionary unique contains only one key—the number that does not repeat. Using list(unique.keys())[0], we convert the dictionary’s keys to a list and retrieve this single number. This approach ensures that we correctly identify the unique number from the array.

This method is efficient and straightforward, leveraging Python’s dictionary and list functionalities to solve the problem effectively.