Create 3583. Count Special Triplets (#954)

Author: Chayandas07

3583. Count Special Triplets

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+class Solution {
+public:
+    int specialTriplets(vector<int>& nums) {
+        const int n = 100001, mod = 1000000007;
+        long ans = 0;
+        int hash[n], prev[n];
+        for(int i = 0; i < n; i++) hash[i] = prev[i] = 0;
+        for(int i = 0; i < nums.size(); i++) hash[nums[i]]++;
+        for(int i = 1; i < nums.size() - 1; i++) {
+            prev[nums[i - 1]]++;
+            int j = nums[i], k = 2*j;
+            if(k < n) {
+                ans += (long)prev[k] * (hash[k] - prev[k] - (j == 0));
+            }
+        }
+        return ans % mod;
+    }
+};