Method add_edge

publication/code/chapter09/cooking_topological_sort.rs:60–75

(&mut self, src: &T, des: &T)

    }

    fn add_edge(&mut self, src: &T, des: &T) {
        if !self.vertices.contains_key(src) { let _sv = self.add_vertex(src); }
        if !self.vertices.contains_key(des) { let _dv = self.add_vertex(des); }

        // 添加点
        self.edgenums += 1;
        self.vertices.get_mut(src)
                     .unwrap()
                     .add_neighbor(des.clone());

        // 添加边
        if !self.edges.contains_key(src) {
            let _eg = self.edges.insert(src.clone(), Vec::new());
        }
        self.edges.get_mut(src).unwrap().push(des.clone());
    }
}

// 构建课程关系图