CF 797, got stuck on E and had to leave, went pretty well

codeforces/rounds/797/e.cpp

@@ -0,0 +1,76 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+void test_case(int tc){
+    int n, k;
+
+    cin >> n >> k;
+
+    vector<vector<int>> modulos(k);
+
+    for(int i = 0; i < n; ++i){
+        int s;
+        cin >> s;
+
+        modulos[s % k].push_back(s);
+    }
+
+    for(auto v : modulos){
+        sort(v.begin(), v.end());
+    }
+
+    unsigned long long result = 0;
+
+    for(int i = k - 1; i > 0; i--){
+        while(!modulos[i].empty() && !modulos[k-i].empty()){
+            int a = modulos[i].back(), b = modulos[k - 1].back();
+            modulos[i].pop_back();
+            modulos[k-1].pop_back();
+
+            result += (a + b) / k;
+        }
+    }
+
+    while(modulos[0].size() > 1){
+        int a = modulos[0].back();
+        modulos[0].pop_back();
+        int b = modulos[0].back();
+        modulos[0].pop_back();
+
+        result += (a + b) / k;
+    }
+
+    for(int i = 0; i < k; ++i){
+        for(int j = i + 1; j < k; ++j){
+            while(!modulos[i].empty() && !modulos[j].empty()){
+                int a = modulos[i].back(), b = modulos[j].back();
+                modulos[i].pop_back();
+                modulos[j].pop_back();
+
+                result += (a + b) / k;
+            }
+        }
+    }
+
+    cout << result << '\n';
+
+}
+
+
+
+int main(){
+    ios::sync_with_stdio(0);
+    cin.tie(0);
+
+    int t;
+    cin >> t;
+
+    for(int tc = 1; tc <= t; ++tc){
+        test_case(tc);
+    }
+
+    cout << flush;
+
+    return 0;
+}