[UVA315]Network（tarjan, 求割点）

题目链接：https://uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=251

求割点，除了输入用strtok和sscanf处理输入以外，对于求割点的tarjan算法有了进一步理解。

特别注意88行，如果u是根并且至少两个儿子，那它一定是割点无误，还有第二个情况用如图代表：

这个例子里显然：low[4]=2,dfn[4]=4，dfn[3]=3。现dfs到3点位置了，4是3的儿子，假如3是割点，那删掉3后4和2依然连通，因此3不是割点。判断依据可以是low[4]<dfn[3]。

假如low[4]>=dfn[3]的话，那3就是割点了。

 /*
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 ┓┏┓┏┓┃キリキリ♂ mind！
 ┛┗┛┗┛┃＼○／
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 ┛┗┛┗┛┃ノ)
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 ┻┻┻┻┻┻
 */
 #include <algorithm>
 #include <iostream>
 #include <iomanip>
 #include <cstring>
 #include <climits>
 #include <complex>
 #include <fstream>
 #include <cassert>
 #include <cstdio>
 #include <bitset>
 #include <vector>
 #include <deque>
 #include <queue>
 #include <stack>
 #include <ctime>
 #include <set>
 #include <map>
 #include <cmath>

 using namespace std;

 #define fr first
 #define sc second
 #define cl clear
 #define BUG puts("here!!!")
 #define W(a) while(a--)
 #define pb(a) push_back(a)
 #define Rint(a) scanf("%d", &a)
 #define Rll(a) scanf("%lld", &a)
 #define Rs(a) scanf("%s", a)
 #define Cin(a) cin >> a
 #define FRead() freopen("in", "r", stdin)
 #define FWrite() freopen("out", "w", stdout)
 #define Rep(i, len) for(int i = 0; i < (len); i++)
 #define For(i, a, len) for(int i = (a); i < (len); i++)
 #define Cls(a) memset((a), 0, sizeof(a))
 #define Clr(a, x) memset((a), (x), sizeof(a))
 #define Full(a) memset((a), 0x7f7f, sizeof(a))
 #define lrt rt << 1
 #define rrt rt << 1 | 1
 #define pi 3.14159265359
 #define RT return
 typedef long long LL;
 typedef long double LD;
 typedef unsigned long long ULL;
 typedef pair<int, int> pii;
 typedef pair<string, int> psi;
 typedef map<string, int> msi;
 typedef vector<int> vi;
 typedef vector<LL> vl;
 typedef vector<vl> vvl;
 typedef vector<bool> vb;

 const int maxn = ;
 char str[];
 int n, m, rt, bcnt;
 int G[maxn][maxn];
 int dfn[maxn], low[maxn], vis[maxn];
 int in[maxn];
 bool cut[maxn];
 vi e[maxn];
 int bridge[maxn][];

 void dfs(int u, int d) {
     int son = ;
     vis[u] = ; dfn[u] = low[u] = d;
     Rep(i, e[u].size()) {
         int v = e[u][i];
         if(!vis[v]) {
             dfs(v, d+); son++;
             low[u] = min(low[u], low[v]);
             if((u==rt&&son>)||(u!=rt&&low[v]>=dfn[u])) cut[u] = ;
         }
         else low[u] = min(low[u], dfn[v]);
     }
 }

 int main() {
     // FRead();
     int u, v;
     while(~Rint(n) && n) {
         Cls(G); Cls(dfn); Cls(low); Cls(in);
         Cls(vis); Cls(bridge); Cls(cut); bcnt = ;
         Rep(i, n+) e[i].cl();
         getchar();
         while(gets(str) && strcmp("", str)) {
             char* p = strtok(str, " ");
             sscanf(p, "%d", &u);
             p = strtok(NULL, " ");
             while(p) {
                 sscanf(p, "%d", &v);
                 p = strtok(NULL, " ");
                 G[u][v] = G[v][u] = ;
             }
         }
         For(i, , n+) {
             For(j, i+, n+) {
                 if(G[i][j]) {
                     e[i].push_back(j);
                     e[j].push_back(i);
                 }
             }
         }
         rt = ;
         int ret = ;
         dfs(, );
         For(i, , n+) if(cut[i]) ret++;
         printf("%d\n", ret);
     }
     RT ;
 }