统计无向图中三角形的个数，复杂度m*sqrt(m).

统计无向图中三角形的个数，复杂度m*sqrt(m).

#include<stdio.h>
#include<vector>
#include<set>
#include<math.h>
#include<algorithm>
using namespace std;
#define LL long long
vector<int> G[];
set<LL> st;
int vis[], link[], out[];
int main(void)
{
    LL ans, sum;
    int n, i, j, m, k, x, y, z, B;
    while(scanf("%d%d", &n, &m)!=EOF)
    {
        B = sqrt(m);
        st.clear();
        for(i=;i<=n;i++)
        {
            G[i].clear();
            vis[i] = out[i] = link[i] = ;
        }
        for(i=;i<=m;i++)
        {
            scanf("%d%d", &x, &y);
            G[x].push_back(y), out[x]++;
            G[y].push_back(x), out[y]++;
            st.insert((LL)x*n+y);
            st.insert((LL)y*n+x);
        }
        ans = ;
        for(i=;i<=n;i++)
        {
            x = i;
            vis[x] = ;
            for(j=;j<G[x].size();j++)
                link[G[x][j]] = x;
            for(j=;j<G[x].size();j++)
            {
                sum = ;
                y = G[x][j];
                if(vis[y])
                    continue;
                if(out[y]<=B)
                {
                    for(k=;k<G[y].size();k++)
                    {
                        z = G[y][k];
                        if(link[z]==x)
                            sum++;
                    }
                }
                else
                {
                    for(k=;k<G[x].size();k++)
                    {
                        z = G[x][k];
                        if(st.find((LL)z*n+y)!=st.end())
                            sum++;
                    }
                }
                ans += sum*(sum-)/;
            }
        }
        printf("%lld\n", ans);
    }
}