HDU 6184
思路：三元环。将无向图转换成有向无环图，详见洛谷日报
#pragma GCC optimize(2)
#pragma GCC optimize(3)
#pragma GCC optimize(4)
#include<bits/stdc++.h>
using namespace std;
#define y1 y11
#define fi first
#define se second
#define pi acos(-1.0)
#define LL long long
//#define mp make_pair
#define pb emplace_back
#define ls rt<<1, l, m
#define rs rt<<1|1, m+1, r
#define ULL unsigned LL
#define pll pair<LL, LL>
#define pli pair<LL, int>
#define pii pair<int, int>
#define piii pair<pii, int>
#define pdd pair<double, double>
#define mem(a, b) memset(a, b, sizeof(a))
#define debug(x) cerr << #x << " = " << x << "
";
#define fio ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
//head
const int N = 2e5 + 5;
vector<int> g[N];
struct Node {int u, v;}e[N];
int n, m, d[N], vis[N], cnt[N];
int main() {
while(~scanf("%d %d", &n, &m)) {
for (int i = 1; i <= m; ++i) scanf("%d %d", &e[i].u, &e[i].v), d[e[i].u]++, d[e[i].v]++;
for (int i = 1; i <= m; ++i) {
if(d[e[i].u] < d[e[i].v]) swap(e[i].u, e[i].v);
if(d[e[i].u] == d[e[i].v] && e[i].u > e[i].v) swap(e[i].u, e[i].v);
g[e[i].u].pb(i);
}
for (int u = 1; u <= n; ++u) {
for (int id : g[u]) {
int to = u^e[id].u^e[id].v;
vis[to] = id;
}
for (int id : g[u]) {
int to = u^e[id].u^e[id].v;
for (int ii : g[to]) {
int tt = to^e[ii].u^e[ii].v;
if(vis[tt]) {
cnt[id]++;
cnt[ii]++;
cnt[vis[tt]]++;
}
}
}
for (int id : g[u]) {
int to = u^e[id].u^e[id].v;
vis[to] = 0;
}
}
LL ans = 0;
for (int i = 1; i <= m; ++i) ans += cnt[i]*1LL*(cnt[i]-1)/2;
printf("%lld
", ans);
for (int i = 1; i <= n; ++i) g[i].clear(), d[i] = 0;
for (int i = 1; i <= m; ++i) cnt[i] = 0;
}
return 0;
}