Method dfs

src/graph/undirected/cycle.rs:47–70

(&mut self, g: &dyn IGraph, v: usize, u: usize)

impl Cycle {
    fn dfs(&mut self, g: &dyn IGraph, v: usize, u: usize) {
        self.marked[v] = true;
        for &w in g.adj(v) {
            // short circuit if cycle already found
            if self.cycle.is_some() {
                return;
            }

            if !self.marked[w] {
                self.edge_to[w] = v;
                self.dfs(g, w, v);
            } else if w != u {
                let mut cycle = Stack::default();
                let mut x = v;
                while x != w {
                    cycle.push(x);
                    x = self.edge_to[x];
                }
                cycle.push(w);
                cycle.push(v);
                self.cycle = Some(cycle);
            }
        }
    }

    // does this graph have two parallel edges?
    fn has_parallel_edges(&mut self, graph: &dyn IGraph) -> bool {