public void dfs(DiGraph g, int s) {
marked[s] = true;
onStack[s] = true;
for (int w : g.adj(s))
if (this.hasCycle()) return;
else if (!marked(w)) {
edgeTo[w] = s;
dfs(g, w);
} else if (onStack[w]) {
cycle = new Stack<Integer>();
for (int x = s; x != w; x = edgeTo[x]) cycle.push(x);
cycle.push(w);
cycle.push(s);
}
onStack[s] = false;
}
