Create 3623. Count Number of Trapezoids I (#947)

Author: Chayandas07

3623. Count Number of Trapezoids I

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+#include <bits/stdc++.h>
+using namespace std;
+
+class Solution {
+public:
+    int countTrapezoids(vector<vector<int>>& points) {
+        const long long MOD = 1000000007LL;
+        const long long INV2 = (MOD + 1) / 2;
+
+        unordered_map<long long, int> freq;
+        freq.reserve(points.size() * 2);
+        for (auto &p : points) {
+            long long y = p[1];
+            ++freq[y];
+        }
+
+        long long sumF = 0;
+        long long sumF2 = 0;
+
+        for (auto &kv : freq) {
+            long long c = kv.second;
+            if (c >= 2) {
+                long long f = c * (c - 1) / 2 % MOD;
+                sumF = (sumF + f) % MOD;
+                sumF2 = (sumF2 + f * f % MOD) % MOD;
+            }
+        }
+
+        long long ans = (sumF * sumF) % MOD;
+        ans = (ans - sumF2 + MOD) % MOD;
+        ans = ans * INV2 % MOD;
+
+        return (int)ans;
+    }
+};