HDU 6184 Counting Stars

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 << "\n";
#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\n", 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;
}