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day2/U111091/1.myans

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day2/U111091/chat.cpp

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+#include <iostream>
+#include <vector>
+
+const int MOD = 1000000007;
+
+// 快速幂计算
+long long mod_pow(long long base, long long exp, long long mod) {
+    long long result = 1;
+    while (exp > 0) {
+        if (exp % 2 == 1) {
+            result = (result * base) % mod;
+        }
+        base = (base * base) % mod;
+        exp /= 2;
+    }
+    return result;
+}
+
+// 计算前缀和
+void compute_prefix_sum(const std::vector<int>& array, std::vector<long long>& prefix) {
+    int n = array.size();
+    prefix[0] = 0;
+    for (int i = 1; i <= n; ++i) {
+        prefix[i] = (prefix[i - 1] + array[i - 1]) % MOD;
+    }
+}
+
+// 计算所有子区间平均数之和
+long long calculate_average_sum(int n, const std::vector<int>& array) {
+    std::vector<long long> prefix(n + 1, 0);
+    compute_prefix_sum(array, prefix);
+    
+    // 预处理每个长度的逆元
+    std::vector<long long> inverse(n + 1);
+    for (int i = 1; i <= n; ++i) {
+        inverse[i] = mod_pow(i, MOD - 2, MOD);
+    }
+    
+    long long total_avg_sum = 0;
+    for (int l = 0; l < n; ++l) {
+        for (int r = l; r < n; ++r) {
+            long long subarray_sum = (prefix[r + 1] - prefix[l] + MOD) % MOD;
+            int length = r - l + 1;
+            long long avg = (subarray_sum * inverse[length]) % MOD;
+            total_avg_sum = (total_avg_sum + avg) % MOD;
+        }
+    }
+    
+    return total_avg_sum;
+}
+
+int main() {
+    int t;
+    std::cin >> t;
+    std::vector<long long> results;
+    
+    while (t--) {
+        int n;
+        std::cin >> n;
+        std::vector<int> array(n);
+        for (int i = 0; i < n; ++i) {
+            std::cin >> array[i];
+        }
+        results.push_back(calculate_average_sum(n, array));
+    }
+    
+    for (long long result : results) {
+        std::cout << result << std::endl;
+    }
+    
+    return 0;
+}