hdu 1269

OJ

#include <iostream>#include <cstdio>#include <cstdlib>#include <algorithm>#include <vector>#include <map>#include <set>#include <string>#include <cstring>#include <list>#include <queue>#include <stack>#include <cmath>using namespace std;#define PF(x) (scanf("%d",&x))#define PT(x,y) (scanf("%d%d",&x,&y))#define PR(x) (printf("%d\n",x))#define PRT(x,y)(printf("%d %d\n",x,y))#define PB(x)(scanf("%I64d",&x))#define PRB(x)(printf("%I64d\n",(x)))#define N 100005#define M 10000#define Mod 1000int n,m,k;int low[N];int start[N];int t;stack<int> G;vector<int> Map[N];int sum = 0;int vis[N];int instack[N];void reinit(){t = 0;sum = 0;while(!G.empty())G.pop();for(int i=1;i<=n;i++)if(!Map[i].empty()) Map[i].clear();memset(vis,0,sizeof(vis));memset(instack,0,sizeof(instack));}inline int minx(int a,int b){return a>b?b:a;}void tarjan(int v){low[v]=start[v] = ++t;vis[v]=1;G.push(v);instack[v] = 1;for(int i=0;i<(int)Map[v].size();i++){if(!vis[Map[v][i]]){tarjan(Map[v][i]);low[v] = minx(low[v],low[Map[v][i]]);}else if(instack[Map[v][i]])low[v] = minx(low[v],low[Map[v][i]]);}if(low[v] == start[v]){sum++;int s;while(!G.empty()){s = G.top();G.pop();instack[s]=0;if(v == s) break;}}}void init(){while(scanf("%d%d",&n,&m)!=EOF &&(m+n)){reinit();for(int i=0;i<m;i++){int x,y;PT(x,y);Map[x].push_back(y);}for(int i=1;i<=n;i++)if(!vis[i]) tarjan(i);if(sum == 1) puts("Yes");else puts("No");}return ;}int main(){init();return 0;}