Debug and Fix Provided Code | Dataford Interview Questions

Identify bugs in a given piece of code and fix them.

Asked in the Technical Interview stage. Code review exercise.

Task

You are given a Python function that is intended to return the first duplicate value in a list of integers, or -1 if no duplicate exists. The current implementation contains bugs.

Write a corrected version of the function with the same signature:

def first_duplicate(nums):

The function must return the first value that appears more than once when scanning from left to right. If no value repeats, return -1.

Constraints

1 <= len(nums) <= 10^5
-10^9 <= nums[i] <= 10^9
Return the first repeated value by scan order, not the most frequent value

Function Signature

def first_duplicate(nums):

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Problem