Method add_neighbor

code/chapter08/word_ladder.rs:70–72

(&mut self, nbr: T, wt: u32)

    }

    fn add_neighbor(&mut self, nbr: T, wt: u32) {
        self.neighbors.push((nbr, wt));
    }

    // 获取邻点
    fn get_neighbors(&self) -> Vec<&T> {