http://acm.sdut.edu.cn/sdutoj/problem.php?action=showproblem&problemid=2138
注意该图为有向图，1000个点应该最多有1000*999条边。在这跪了一下。。。
1 #include<stdio.h> 2 #include<string.h> 3 #include<queue> 4 5 using namespace std; 6 7 const int maxn=1000002; 8 int head[maxn],vis[maxn]; 9 int n,m,cnt; 10 struct node 11 { 12 int u; 13 int v; 14 int next; 15 } edge[maxn]; 16 void add(int u,int v) 17 { 18 edge[cnt].u = u; 19 edge[cnt].v = v; 20 edge[cnt].next = head[u]; 21 head[u] = cnt++; 22 } 23 int bfs(int s) 24 { 25 26 queue<int>q; 27 q.push(s); 28 vis[s] = 1; 29 while(!q.empty()) 30 { 31 int u = q.front(); 32 if (u==1) 33 return 1; 34 q.pop(); 35 for (int j = head[u]; j!=-1; j = edge[j].next) 36 { 37 int v = edge[j].v; 38 if (!vis[v]) 39 { 40 vis[v] = 1; 41 q.push(v); 42 } 43 } 44 } 45 return 0; 46 } 47 int main() 48 { 49 while(~scanf("%d%d",&n,&m)) 50 { 51 int u,v; 52 cnt = 0; 53 memset(vis,0,sizeof(vis)); 54 memset(head,-1,sizeof(head)); 55 for (int i = 0; i < m; i ++) 56 { 57 scanf("%d%d",&u,&v); 58 add(u,v); 59 } 60 if(bfs(n)) 61 printf("YES "); 62 else 63 printf("NO "); 64 } 65 return 0; 66 }