P3799 妖梦拼木棒

欲由4根木棒组成一个正三角形，则必有2根长度相等，且另外2根长度之和，等于前2根相等的木棒的长度。

#include <cstdio>
#include <algorithm>

using LL = long long;

const int N = 1e5 + 5, mod = 1e9 + 7;

int n, a[N], cnt[N];

int main()
{
	scanf("%d", &n);
	int min = 1e9, max = -1e9;
	for (int i = 0; i < n; i++) {
		scanf("%d", &a[i]);
		cnt[a[i]]++;
		min = std::min(min, a[i]);
		max = std::max(max, a[i]);
	}
	
	LL ans = 0;
	for (int i = min; i <= max; i++) {
		if (cnt[i] >= 2) {
		    LL times = cnt[i] * (cnt[i] - 1) / 2 % mod;
			for (int j = min; j <= i / 2; j++) {
				if (j != i - j && cnt[j] >= 1 && cnt[i - j] >= 1) ans += times * cnt[j] * cnt[i - j] % mod;
				else if (j == i - j && cnt[j] >= 2) ans += times * (cnt[j] * (cnt[j] - 1) / 2) % mod;
			}
		}
	}
	printf("%lld", ans%mod);
	return 0;
}