First Unique Element Index

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Your question is First Unique Element Index. Start with the requirements on the right.

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Problem

Given an array of integers nums, return the index of the first element that appears exactly once. If no such element exists, return -1. The input is a Python list of integers, and the output is a single integer index.

Constraints

1 <= len(nums) <= 10^5
-10^9 <= nums[i] <= 10^9
An O(n) solution is expected

Function Signature

def first_unique_index(nums):

Your solutionPython 3

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