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pojcs101_M03441_sum_zero.cpp

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+#include <iostream>
+#include <vector>
+#include <unordered_map>
+#include <algorithm>
+
+using namespace std;
+
+constexpr size_t MAX_N = 4005;
+long long numbers_amount, numbers[4][MAX_N];
+vector<long long> sums[2];
+unordered_map<long long, size_t> sum_to_amount[2];
+
+template <typename T>
+void log_vector(vector<T> vec, const string& prefix = "") {
+    clog << prefix;
+    for (const T& n : vec) {
+        clog << n << " ";
+    }
+    clog << endl;
+}
+
+void build_sums(const size_t sums_idx) {
+    for (int i = 0; i < numbers_amount; i++) {
+        for (int j = 0; j < numbers_amount; j++) {
+            long long sum = numbers[sums_idx][i] + numbers[sums_idx + 1][j];
+            if (!sum_to_amount[sums_idx / 2][sum]++) {
+                sums[sums_idx / 2].push_back(sum);
+            }
+        }
+    }
+}
+
+int main() {
+    ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    cin >> numbers_amount;
+    for (int i = 0; i < numbers_amount; i++) {
+        cin >> numbers[0][i] >> numbers[1][i] >> numbers[2][i] >> numbers[3][i];
+    }
+    build_sums(0), build_sums(2);
+    sort(sums[0].begin(), sums[0].end()), sort(sums[1].begin(), sums[1].end());
+    // log_vector(sums[0], "sums[0]: ");
+    // log_vector(sums[1], "sums[1]: ");
+
+    unsigned int ans = 0;
+    for (const long long n : sums[0]) {
+        auto it = lower_bound(sums[1].begin(), sums[1].end(), -n);
+        if (it != sums[1].end() && *it == -n) {
+            ans += sum_to_amount[1][-n] * sum_to_amount[0][n];
+        }
+    }
+    cout << ans << endl;
+
+    return 0;
+}