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[100005];  
set<LL> st;  
int vis[100005], link[100005], out[100005];  
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=1;i<=n;i++)  
        {  
            G[i].clear();  
            vis[i] = out[i] = link[i] = 0;  
        }  
        for(i=1;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 = 0;  
        for(i=1;i<=n;i++)  
        {  
            x = i;  
            vis[x] = 1;  
            for(j=0;j<G[x].size();j++)  
                link[G[x][j]] = x;  
            for(j=0;j<G[x].size();j++)  
            {  
                sum = 0;  
                y = G[x][j];  
                if(vis[y])  
                    continue;  
                if(out[y]<=B)  
                {  
                    for(k=0;k<G[y].size();k++)  
                    {  
                        z = G[y][k];  
                        if(link[z]==x)  
                            sum++;  
                    }  
                }  
                else  
                {  
                    for(k=0;k<G[x].size();k++)  
                    {  
                        z = G[x][k];  
                        if(st.find((LL)z*n+y)!=st.end())  
                            sum++;  
                    }  
                }  
                ans += sum*(sum-1)/2;  
            }  
        }  
        printf("%lld
", ans);  
    }  
}