Explain Why Code Is Wrong and How to Improve It

Given a piece of code, explain why it is wrong and how it could be improved.

Asked in the technical interview stage. Reported by a hired USAA SWE as part of their interview.

Task

You are given a Python function that is intended to return the first duplicate value in a list, or -1 if no duplicate exists.

def first_duplicate(nums):
    seen = []
    for i in range(len(nums)):
        if nums[i] in seen:
            return nums[i]
        seen.append(nums[i])
    return 0

Write a corrected version of the function and explain what is wrong with the original code.

Input

• nums: a list of integers

Output

• Return the first value that appears more than once, or -1 if there is no duplicate.

Requirements

1. Identify the bug in the return value.
2. Improve the implementation for better performance.
3. Preserve the intended behavior of returning the first duplicate encountered during a left-to-right scan.