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Leetcode 3583. Count Special Triplets
Count the number of index triplets (i<j<k) where nums[i] and nums[k] both equal 2 * nums[j]; this reduces to, for each j, multiplying the count of 2*nums[j] on the left by the count on the right and summing (mod 1e9+7). Constraints: n up to 1e5 and nums[i] up to 1e5, so use frequency maps/prefix-suffix counts for an O(n) solution.
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