First Repeat in C#

Given an array of integers nums, read it from left to right and return the first value that appears for the second time.

If no value appears twice, return -1.

Example 1:

Input: nums = [3, 1, 4, 1, 3]
Output: 1
Explanation: Reading from the left: 3, 1, 4, then 1 again. The second 1 (index 3) comes before the second 3 (index 4), so the answer is 1, although 3 showed up first.

Example 2:

Input: nums = [2, 7, 9]
Output: -1
Explanation: Every value appears once, so nothing repeats.

Example 3:

Input: nums = [6, 6, 6, 6]
Output: 6
Explanation: The second 6, at index 1, is already a repeat.

Constraints:

• nums has at least one number.
• Every number is between 0 and 10⁹, so -1 always means that no value repeats.