NEUOJ 1176 Problem: Easy to improve
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#include <iostream>using namespace std;const int Mod = 1e9+7;const int maxn = 1e5+7;long long a[maxn] = {0};int main() { int T; cin >> T; while(T--) { int n; cin >> n; for(int i = 0; i < n; i++) cin >> a[i]; long long x, y, z, ans = 0; x = y = z = 0; for(int i = 0; i < n; i++) { x += a[i]; y += a[i]*a[i]; z += a[i]*a[i]*a[i]; x %= Mod; y %= Mod; z %= Mod; } ans = (n-4)*z + 3*x*y; ans %= Mod; cout << ans << endl; } return 0;}/*将(ai+aj)^3拆开,再整理到一起可得结果。表面上看a[i]<=100000, 但是在计算x时a[i]*a[i]爆了int,我认为计算a*a时将结果存在了一个temp中,而temp的类型与a[i]相同。*/
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