http://oj.xjtuacm.com/problem/12/

題解點這裡~~~

#include<stdio.h>
#include<cstring>
#include<vector>
using namespace std;
typedef long long ll;
#define pb push_back
#define N 100005
#define M 200005

vector<int> G1[N],G2[N],G[N];
int sum1[N],sum2[N],sum3[M],n,m,du[N],e[M][2];

#define For(i,v) for (int _##i=0,i;_##i<v.size() && (i=v[_##i]);_##i++)
 


inline ll sum_4()
{
    ll ans1=0,ans2=0;
    for (int i=1;i<=n;i++)
    {
        For(j,G1[i]) For(k,G1[j]) sum1[k]++;
        For(j,G2[i]) For(k,G1[j]) sum2[k]++;
        For(j,G1[i]) For(k,G1[j])
        {
            ans1+=(ll)sum1[k]*(sum1[k]-1)/2;
            ans1+=(ll)sum1[k]*sum2[k];
            sum1[k]=0
 
;
        }
        For(j,G2[i])For(k,G1[j])
        {
            if (k!=i) ans2+=(ll)sum2[k]*(sum2[k]-1)/2;
            sum2[k]=0;
        }
    }
    return ans1+ans2/2;
}
inline ll sum_3()
{
    for (int i=1;i<=m;i++)
    {
        int u=e[i][0],v=e[i][1];
        For(j,G1[u]) sum1[j]=G[u][_j];
        For(j,G1[v]) if
 
 (sum1[j])
        {
            sum3[i]++;
            sum3[sum1[j]]++;
            sum3[G[v][_j]]++;
        }
        For(j,G1[u]) sum1[j]=0;
    }
    ll ans=0;
    for (int i=1;i<=m;i++) ans+=(ll)sum3[i]*(sum3[i]-1)/2;
    return ans;
}


int main()
{
    while (~scanf("%d%d",&n,&m))
    {
        for (int i=1;i<=n;i++) G1[i].clear(),G2[i].clear(),G[i].clear();
        memset(du+1,0,n*sizeof(int));
        memset(sum3+1,0,m*sizeof(int));
        for (int i=1,u,v;i<=m;i++)
        {
            scanf("%d%d",&u,&v);
            e[i][0]=u,e[i][1]=v;
            du[u]++,du[v]++;
        }
        for (int i=1;i<=m;i++)
        {
            int u=e[i][0],v=e[i][1];
            if (du[v]<du[u] || (du[u]==du[v] && v<u)) swap(u,v);
            G1[u].pb(v);G2[v].pb(u);
            G[u].pb(i);
        }
        printf("%lld\n",2*sum_4()-sum_3()); 
    }
}