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。
1 #include <cstdio> 2 #include <algorithm> 3 #include <iostream> 4 #include <cstring> 5 #include <string> 6 #include <cmath> 7 #include <queue> 8 #include <vector> 9 #include <stack> 10 using namespace std; 11 #define M 100010 12 #define N 10010 13 struct node 14 { 15 int v, next; 16 }edge[M]; 17 int n, m, tot, cnt, head[N]; 18 int dfn[N], low[N]; 19 bool vis[N]; 20 stack<int> sta; 21 22 void init() 23 { 24 memset(dfn, 0, sizeof(dfn)); 25 memset(low, 0, sizeof(low)); 26 memset(head, -1, sizeof(head)); 27 memset(vis, false, sizeof(vis)); 28 tot = cnt = 0; 29 while(!sta.empty()) sta.pop(); 30 } 31 32 void add(int u, int v) 33 { 34 edge[tot].next = head[u]; edge[tot].v = v; head[u] = tot++; 35 } 36 37 int tarjan(int u) 38 { 39 dfn[u] = low[u] = ++cnt; 40 sta.push(u); 41 vis[u] = true; 42 for(int i = head[u]; ~i; i = edge[i].next) { 43 int v = edge[i].v; 44 if(!dfn[v]) { 45 tarjan(v); 46 if(low[v] < low[u]) low[u] = low[v]; 47 } else { 48 if(dfn[v] < low[u] && vis[v]) low[u] = dfn[v]; 49 } 50 } 51 if(low[u] == dfn[u]) { 52 int top = 0; 53 int ans = 0; 54 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 55 ans++; 56 top = sta.top(); sta.pop(); 57 vis[top] = false; 58 } 59 return ans; 60 } 61 } 62 63 int main() 64 { 65 while(scanf("%d%d", &n, &m)) { 66 if(n + m == 0) break; 67 init(); 68 for(int i = 0; i < m; i++) { 69 int u, v; 70 scanf("%d%d", &u, &v); 71 add(u, v); 72 } 73 int ans = tarjan(1); 74 if(n == ans) puts("Yes"); 75 else puts("No"); 76 } 77 return 0; 78 }