388. Count Unique Elements Frequency

Given an array of integers nums, return the count of how many distinct numbers appear exactly the same number of times as their value. For example, if the number 3 appears exactly 3 times, it contributes to the final count. If the number 5 appears 2 times, it does not contribute.

Input: nums = [2, 2, 3, 3, 3, 4]
Output: 2 (Because 2 appears twice, and 3 appears three times.)

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