poj2762 Going from u to v or from v to u?

const int maxn = 1010;
int n, m;
vector<vector<int> > G;
int dfn[maxn], low[maxn], depth;
int belong[maxn], block;
int in[maxn], mark[maxn];
stack<int> st;
int head[maxn], pnt[maxn<<3], nxt[maxn<<3], ecnt;
void addedge(int u,int v) {
pnt[ecnt] = v, nxt[ecnt] = head[u], head[u] = ecnt++;
}
void tarjan(int u) {
dfn[u] = low[u] = ++depth;
st.push(u), mark[u] = 1;
int Size = G[u].size();
for (int i = 0;i < Size;++i) {
int v = G[u][i];
if (dfn[v] == 0) {
tarjan(v);
low[u] = min(low[v], low[u]);
}else if (mark[v]) low[u] = min(low[u], dfn[v]);
}

if (dfn[u] == low[u]) {
block++;
while(true) {
int x = st.top();
st.pop();
belong[x] = block;
mark[x] = 0;
if (x == u) break;
}
}

return ;
}
int ans;
void dfs(int u) {
ans++;
mark[u] = 1;
for (int i = head[u]; ~i;i = nxt[i]) {
int v = pnt[i];
if (!mark[v]) {
// ans++;
dfs(v);
return ;
}
}
}
int main(int argc, const char * argv[])
{
// freopen("in.txt","r",stdin);
// freopen("out.txt","w",stdout);
// clock_t _ = clock();

int t;
cin >> t;
while(t--) {
scanf("%d%d", &n, &m);
G.clear();G.resize(n + 2);
int u, v;
while(m--) {
scanf("%d%d", &u, &v);
G[u].push_back(v);
}
memset(dfn, 0,sizeof dfn), depth = block = 0;
memset(mark, 0,sizeof mark);
for (int i = 1;i <= n;++i)
if (dfn[i] == 0) tarjan(i);

memset(head, -1,sizeof head), ecnt = 0;
memset(in, 0,sizeof in);
for (int i = 1;i <= n;++i) {
for (int j = 0;j < G[i].size();++j) {
u = i, v = G[i][j];
if (belong[u] != belong[v]) {
addedge(belong[u], belong[v]);
in[belong[v]]++;
}
}
}
ans = 0;
for (int i = 1;i <= block;++i)
if (in[i] == 0) {dfs(i);break;}
if (ans == block) puts("Yes");
else puts("No");
}

// printf("\nTime cost: %.2fs\n", 1.0 * (clock() - _) / CLOCKS_PER_SEC);
return 0;
}