迷宫城堡

3 3
1 2
2 3
3 1
3 3
1 2
2 3
3 2
0 0

Yes
No

#include <cstdio>
#include <cstring>
#include <algorithm>
#define maxn 10000 + 100
#define maxm 100000 + 1000
using namespace std;
int n, m;
struct node {
	int u, v, next;
};
node edge[maxm];
int head[maxn], cnt;
int low[maxn], dfn[maxn];
int dfs_clock;
int Stack[maxn];
bool Instack[maxn];
int top;
int Belong[maxn] , scc_clock;

void init(){
	cnt = 0;
	memset(head, -1, sizeof(head));
}

void addedge(int u, int v){
	edge[cnt] = {u, v, head[u]};
	head[u] = cnt++;
}

void getmap(){
    while(m--){
        int a, b;
        scanf("%d%d", &a, &b);
        addedge(a, b);
    }
}

void tarjan(int u, int per){
    int v;
    low[u] = dfn[u] = ++dfs_clock;
    Stack[top++] = u;
    Instack[u] = true;
    int have = 1;
    for(int i = head[u]; i != -1; i = edge[i].next){
        v = edge[i].v;
        if(v == per && have){
            have = 0;
            continue;
        }
        if(!dfn[v]){
            tarjan(v, u);
            low[u] = min(low[v], low[u]);
        }
        else if(Instack[v]){
            low[u] = min(low[u], dfn[v]);
        }
    }
    if(dfn[u] == low[u]){
        scc_clock++;
        do{
            v = Stack[--top];
            Instack[v] = false;
            Belong[v] = scc_clock;
        }while(u != v);
    }
}

void find(){
    memset(low, 0, sizeof(low));
    memset(dfn, 0, sizeof(dfn));
    memset(Instack, false, sizeof(Instack));
    memset(Belong, 0, sizeof(Belong));
    dfs_clock = scc_clock = top = 0;
    for(int i = 1; i <= n; ++i){
        if(!dfn[i])
            tarjan(i, i);
    }
}

void solve(){
    if(scc_clock == 1)
        printf("Yes\n");
    else
        printf("No\n");
}

int main (){
	while(scanf("%d%d", &n, &m), n || m){
        init();
        getmap();
        find();
        solve();
	}
	return 0;
}