三元环计数问题

参考博客

洛谷日报

代码（[HDU6184]Counting Stars）

#include <bits/stdc++.h>
#define rin(i,a,b) for(register int i=(a);i<=(b);++i)
#define irin(i,a,b) for(register int i=(a);i>=(b);--i)
#define trav(i,a) for(register int i=head[a];i;i=e[i].nxt)
typedef long long LL;
using std::cin;
using std::cout;
using std::endl;

inline int read(){
    int x=0,f=1;char ch=getchar();
    while(!isdigit(ch)){if(ch=='-')f=-1;ch=getchar();}
    while(isdigit(ch)){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}

const int MAXN=300005;
int n,m,ecnt,head[MAXN],uu[MAXN<<1],vv[MAXN<<1];
int deg[MAXN],vis[MAXN],id[MAXN],cnt[MAXN];
struct Edge{
    int to,nxt;
}e[MAXN<<1];

inline void add_edge(int bg,int ed){
    ecnt++;
    e[ecnt].to=ed;
    e[ecnt].nxt=head[bg];
    head[bg]=ecnt;
}

int main(){
    while(~scanf("%d%d",&n,&m)){
        ecnt=0;
        rin(i,1,n) head[i]=deg[i]=vis[i]=0;
        rin(i,1,m){
            cnt[i]=0;
            uu[i]=read(),vv[i]=read();
            ++deg[uu[i]],++deg[vv[i]];
        }
        rin(i,1,m){
            int u=uu[i],v=vv[i];
            if(deg[u]>deg[v]||(deg[u]==deg[v]&&u<v)) add_edge(u,v);
            else add_edge(v,u);
        }
        rin(i,1,n){
            trav(j,i) vis[e[j].to]=i,id[e[j].to]=j;
            trav(j,i){
                int ver=e[j].to;
                trav(k,ver){
                    int ver2=e[k].to;
                    if(vis[ver2]==i) ++cnt[j],++cnt[k],++cnt[id[ver2]];
                }
            }
        }
        LL ans=0;
        rin(i,1,m) ans+=1ll*cnt[i]*(cnt[i]-1)/2;
        printf("%lld\n",ans);
    }
    return 0;
}