POJ 3352 Road Construction

要使游客不受道路施工的影响，能够到达各个景点，那么就要使图是至少一个双连通分量，因为

施工时只对一道路施工。

这样模型就建立了： 给定一个连通的无向图G，至少要添加几条边，才能使其变为双连通图。

同样用tarjan缩点，然后求出缩点树的叶子结点个数，再按照公式计算。

/*Accepted    220K    0MS    C++    1783B    2012-07-30 09:53:31*/
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<algorithm>
using namespace std;

const int MAXN = 1 << 10;
const int MAXR = 1 << 11;
int low[MAXN], first[MAXN], next[MAXR], v[MAXR], s[MAXN], color[MAXN];
int dfn[MAXN], N, R, e, top, cnt, col, dgr[MAXN], ans;

void addedge(int x, int y)
{
    v[e] = y;
    next[e] = first[x], first[x] = e ++;
}

void tarjan(int cur, int p)
{
    int i;
    low[cur] = dfn[cur] = ++ cnt;
    s[top ++] = cur;
    for(i = first[cur]; i != -1; i = next[i])
    {
        if(v[i] != p)
        {
            if(!dfn[v[i]])
            {
                tarjan(v[i], cur);
                if(low[v[i]] < low[cur])
                    low[cur] = low[v[i]];
            }
            else if(dfn[v[i]] < low[cur])
                low[cur] = dfn[v[i]];
        }
    }
    if(low[cur] == dfn[cur])
    {
        ++ col;
        for(s[top] = -1; s[top] != cur; )
            color[s[-- top]] = col;
    }
}

void cal()
{
    int i, j, k;
    ans = 0;
    col = cnt = top = 0;
    memset(dfn, 0, sizeof dfn);
    tarjan(1, -1);
    memset(dgr, 0, sizeof dgr);
    for(i = 1; i <= N; i ++)
        for(j = first[i]; j != -1; j = next[j])
            if(color[i] != color[v[j]])
                ++ dgr[color[i]], ++ dgr[color[v[j]]];
    for(i = 1; i <= col; i ++)
        if(dgr[i] == 2)
            ++ ans;
    ans = (ans + 1) / 2;
}

void ReadGraph()
{
    int i, x, y;
    memset(first, -1, sizeof first);
    e = 0;
    for(i = 0; i < R; i ++)
    {
        scanf("%d%d", &x, &y);
        addedge(x, y), addedge(y, x);
    }
}

int main()
{
    while(scanf("%d%d", &N, &R) == 2)
    {
        ReadGraph();
        cal();
        printf("%d\n", ans);
    }
}