3583. Count Special Triplets

You are given an integer array nums.

A special triplet is defined as a triplet of indices (i, j, k) such that:

• 0 <= i < j < k < n, where n = nums.length
• nums[i] == nums[j] * 2
• nums[k] == nums[j] * 2

Return the total number of special triplets in the array.

Since the answer may be large, return it modulo 109 + 7.

class Solution {
public:
    int specialTriplets(vector<int>& nums) {
        unordered_map<int,long long> freq;
        long long n = nums.size(), mod = 1e9 + 7, res = 0;
        vector<long long> le(n);
        for(int i = 0; i < nums.size(); i++) {
            le[i] = freq[nums[i] * 2];
            freq[nums[i]]++;
        }
        freq = {};
        for(int i = n - 1; i >= 0; i--) {
            res = (res + freq[nums[i] * 2] * le[i] % mod) % mod;
            freq[nums[i]]++;
        }
        return res;
    }
};