有向图的强连通分量（Tarjan）

const int maxn = 100;
vector<int> G[maxn];
int pre[maxn], lowlink[maxn], sccno[maxn], dfs_clock, scc_cnt;
stack<int> S;

void dfs(int u) {
    pre[u] = lowlink[u] = ++dfs_clock;
    S.push(u);
    for(int i = 0; i < G[u].size(); i++){
        int v = G[u][i];
        if(!pre[v]) {
            dfs(v);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        } else if(!sccno[v]) {
            lowlink[u] = min(lowlink[u], pre[v]);
        }
    }
    if(lowlink[u] == pre[u]){
        scc_cnt++;
        for(;;) {
            int x = S.top();S.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}

void find_scc(int n) {
    dfs_clock = scc_cnt = 0;
    memset(sccno, 0, sizeof(sccno));
    memset(pre, 0, sizeof(pre));
    for(int i = 0; i < n; i++){
        if(!pre[i]) dfs(i);
    }
}

#include<bits/stdc++.h>
using namespace std;

const int maxn = 10000 + 5, inf = 1e8;
vector<int> G[maxn];
int pre[maxn], lowlink[maxn], sccno[maxn], dfs_clock, scc_cnt;
stack<int> S;

void dfs(int u) {
    pre[u] = lowlink[u] = ++dfs_clock;
    S.push(u);
    for(int i = 0; i < G[u].size(); i++){
        int v = G[u][i];
        if(!pre[v]) {
            dfs(v);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        } else if(!sccno[v]) {
            lowlink[u] = min(lowlink[u], pre[v]);
        }
    }
    if(lowlink[u] == pre[u]){
        scc_cnt++;
        for(;;) {
            int x = S.top();S.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}

void find_scc(int n) {
    dfs_clock = scc_cnt = 0;
    memset(sccno, 0, sizeof(sccno));
    memset(pre, 0, sizeof(pre));
    for(int i = 1; i <= n; i++){
        if(!pre[i]) dfs(i);
    }
}

int main() {
    int N, M, a, b;
    while(scanf("%d%d", &N, &M) && N != 0){
        for(int i = 1; i <= N; i++) G[i].clear();
        for(int i = 0; i < M; i++) scanf("%d%d", &a, &b),G[a].push_back(b);
        find_scc(N);
        if(scc_cnt == 1) cout<<"Yes\n";
        else cout<<"No\n";
    }
}