CF1418G Three Occurrences

You are given an array $ a $ consisting of $ n $ integers. We denote the subarray $ a[l..r] $ as the array $ [a_l, a_{l + 1}, \dots, a_r] $ ( $ 1 \le l \le r \le n $ ). A subarray is considered good if every integer that occurs in this subarray occurs there exactly thrice. For example, the array $ [1, 2, 2, 2, 1, 1, 2, 2, 2] $ has three good subarrays: - $ a[1..6] = [1, 2, 2, 2, 1, 1] $ ; - $ a[2..4] = [2, 2, 2] $ ; - $ a[7..9] = [2, 2, 2] $ . Calculate the number of good subarrays of the given array $ a $ .

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