ИИ - Алгоритми 2021 задачи за решавање: Двобојност

#include <bits/stdc++.h>
using namespace std;
enum vertex_colour // the colour
{
    UNDEFINED, // non-visited
    RED,
    BLACK
};
struct vertex // a vertex point
{
    vertex_colour colour;
    vector <int> edges_connected;
    vertex()
    {
        colour = UNDEFINED;
        return;
    }
};
vertex_colour opposite(vertex_colour c)
{
    switch (c)
    {
    case RED:
        return BLACK;
    case BLACK:
        return RED;
    case UNDEFINED:
        return UNDEFINED;
    default:
        return UNDEFINED;
    }
}
int main()
{
    int vertices_count, edges; // size of vertices array
    cin >> vertices_count >> edges;
    vector <vertex> vertices(vertices_count); // vertices
    for (int i = 0; i < edges; i++) // input edges
    {
        int a, b;
        cin >> a >> b;
        vertices[a].edges_connected.push_back(b);
        vertices[b].edges_connected.push_back(a);
    }
    queue <int> bfs_queue; // BFS queue
    bfs_queue.push(0); // add vertex 0 to BFS queue
    vertices[0].colour = RED;
    while (!bfs_queue.empty()) // BFS
    {
        int vertex_index = bfs_queue.front(); // get check index for vertex
        bfs_queue.pop();
        for (int i = 0; i < vertices[vertex_index].edges_connected.size(); i++) // check all edges connecting to vertex
        {
            if (vertices[vertices[vertex_index].edges_connected[i]].colour == vertices[vertex_index].colour)
            {
                cout << "NO"; // two neighbour vertices have same colour
                return 0;
            }
            if (vertices[vertices[vertex_index].edges_connected[i]].colour == UNDEFINED)
            {
                vertices[vertices[vertex_index].edges_connected[i]].colour = opposite(vertices[vertex_index].colour);
                bfs_queue.push(vertices[vertex_index].edges_connected[i]); // colour with opposite colour
            }
        }
    }
    cout << "YES"; // all are coloured
    return 0;
}