[无向图割点] PKU 1523 SPF

targan算法。

# include <cstdio>
# include <cstring>

# define N (1000 + 5)

int n, son, tmpdfn;
int low[N], dfn[N], subnets[N];
char g[N][N];

int Min(int x, int y)
{
    return x<y ? x:y;
}

void tarjan(int u, int r)
{
    dfn[u] = low[u] = ++tmpdfn;
    for (int v = 1; v <= n; ++v) if (g[u][v])
    {
        if (!dfn[v])
        {
            //dfn[v] = low[v] = ++tmpdfn;  // 时间戳打在这里就忽略了节点r，原图可能是不联通的（这道题是联通的）
            tarjan(v, r);
            low[u] = Min(low[u], low[v]);
            if (low[v] >= dfn[u])
            {
                if (u != r) ++subnets[u];
                else ++son;
            }
        }
        else
            low[u] = Min(low[u], dfn[v]);
    }
}

void init(void)
{
    tmpdfn = 0;
    memset(low+1, 0, sizeof(int)*n);
    memset(dfn+1, 0, sizeof(int)*n);
    memset(subnets+1, 0, sizeof(int)*n);
}

void solve(void)
{
    bool find = false;
    for (int i = 1; i <= n; ++i) if (!dfn[i])
    {
        son = 0;
        tarjan(i, i);
        if (son > 1) subnets[i] = son - 1;
    }
    for (int i = 1; i <= n; ++i)
    {
        if (subnets[i])
        {
            find = true;
            printf("\n  SPF node %d leaves %d subnets", i, subnets[i]+1);
        }
    }
    if (find == false)
        printf("\n  No SPF nodes");
}

int read_graph(void)
{
    n = 0;
    memset(g, 0, sizeof(g));
    int x, y;
    scanf("%d", &x);
    if (x == 0) return 0;
    do
    {
        scanf("%d", &y);
        g[x][y] = g[y][x] = 1;
        if (x > n) n = x;
        if (y > n) n = y;
    }while (scanf("%d", &x), x);    
    return 1;
}

int main()
{    
    int icase = 0;
    while (read_graph())
    {
        if (icase != 0) printf("\n\n");
        printf("Network #%d", ++icase);
        init();
        solve();
    }
}

注意n=1的情况。