HDU 1269：迷宫城堡（强连通）

http://acm.hdu.edu.cn/showproblem.php?pid=1269

题意：确定是否是一个强连通图。

思路：裸的tarjan算法。

 #include <cstdio>
 #include <algorithm>
 #include <iostream>
 #include <cstring>
 #include <string>
 #include <cmath>
 #include <queue>
 #include <vector>
 #include <stack>
 using namespace std;
 #define M 100010
 #define N 10010
 struct node
 {
     int v, next;
 }edge[M];
 int n, m, tot, cnt, head[N];
 int dfn[N], low[N];
 bool vis[N];
 stack<int> sta;

 void init()
 {
     memset(dfn, , sizeof(dfn));
     memset(low, , sizeof(low));
     memset(head, -, sizeof(head));
     memset(vis, false, sizeof(vis));
     tot = cnt = ;
     while(!sta.empty()) sta.pop();
 }

 void add(int u, int v)
 {
     edge[tot].next = head[u]; edge[tot].v = v; head[u] = tot++;
 }

 int tarjan(int u)
 {
     dfn[u] = low[u] = ++cnt;
     sta.push(u);
     vis[u] = true;
     for(int i = head[u]; ~i; i = edge[i].next) {
         int v = edge[i].v;
         if(!dfn[v]) {
             tarjan(v);
             if(low[v] < low[u]) low[u] = low[v];
         } else {
             if(dfn[v] < low[u] && vis[v]) low[u] = dfn[v];
         }
     }
     if(low[u] == dfn[u]) {
         int top = ;
         int ans = ;
         while(top != u) { //注意一开始top等于u的情况这样ans会只算0,而不是1
             ans++;
             top = sta.top(); sta.pop();
             vis[top] = false;
         }
         return ans;
     }
 }

 int main()
 {
     while(scanf("%d%d", &n, &m)) {
         if(n + m == ) break;
         init();
         for(int i = ; i < m; i++) {
             int u, v;
             scanf("%d%d", &u, &v);
             add(u, v);
         }
         int ans = tarjan();
         if(n == ans) puts("Yes");
         else puts("No");
     }
     return ;
 }