dfs in ford-fulkerson

    fn dfs(&mut self, v: usize, t: usize, f: u32) -> u32 {
        if v == t { return f; }
        self.used[v] = true;
        for e in &mut self.g[v] {
            if !self.used[e.to] && e.cap > 0 {
                let d = self.dfs(e.to, t, std::cmp::min(f, e.cap)); // cannot borrow `*self` as mutable more than once at a time
                if d > 0 {
                    e.cap -= d;
                    self.g[e.to][e.rev].cap += d; // cannot borrow `self.g` as mutable more than once at a time
                    return d;
                }
            }
        }
        return 0;
    }