Раскраска в три цвета

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

using Vertex = size_t;

class Graph {
public:
    vector<vector<Vertex>> graph;
    vector<int> tin;
    vector<int> tout;
    int time;

    explicit Graph(size_t n) {
        graph.resize(n);
        tin.resize(n);
        tout.resize(n);
        time = 0;
    }

    size_t size() const {
        return graph.size();
    }

    void AddEdge(Vertex vert1, Vertex vert2) {
        graph[vert1].push_back(vert2);
    }

    vector<Vertex> GetNeighbours(Vertex vert) {
        return graph[vert];
    }

    void ConnDFS(Vertex vertex, vector<bool> &visited, vector<Vertex> &topologysort) {
        visited[vertex] = true;
        for (Vertex neig: graph[vertex]) {
            if (!visited[neig]) {
                ConnDFS(neig, visited, topologysort);
            }
        }
        topologysort.push_back(vertex);
    }

    void ColourDFS(Vertex vertex, vector<int> &visited, int colour) {
        visited[vertex] = colour;
        for (Vertex neig: graph[vertex]) {
            if (visited[neig] == 0) {
                ColourDFS(neig, visited, colour);
            }
        }

    }

};

int main() {
    int n, m;
    cin >> n >> m;
    string s;
    cin >> s;
    Graph graph(2 * n);
    Graph transgraph(2 * n);
    vector<pair<char, char>> colours(n);
    for (int i = 0; i < n; ++i) {
        if (s[i] == 'R') {
            colours[i].first = 'G';
            colours[i].second = 'B';
        } else if (s[i] == 'G') {
            colours[i].first = 'R';
            colours[i].second = 'B';
        } else {
            colours[i].first = 'R';
            colours[i].second = 'G';
        }
    }
    for (int i = 0; i < m; ++i) {
        int v1, v2;
        cin >> v1 >> v2, --v1, --v2;
        if (colours[v1].first == colours[v2].first) {
            graph.AddEdge(v1 + n, v2);
            graph.AddEdge(v2 + n, v1);
            transgraph.AddEdge(v2, v1 + n);
            transgraph.AddEdge(v1, v2 + n);
        } if (colours[v1].first == colours[v2].second) {
            graph.AddEdge(v2, v1);
            graph.AddEdge(v1 + n, v2 + n);
            transgraph.AddEdge(v1, v2);
            transgraph.AddEdge(v2 + n, v1 + n);
        } if (colours[v1].second == colours[v2].first) {
            graph.AddEdge(v1, v2);
            graph.AddEdge(v2 + n, v1 + n);
            transgraph.AddEdge(v2, v1);
            transgraph.AddEdge(v1 + n, v2 + n);
        } if (colours[v1].second == colours[v2].second){
            graph.AddEdge(v2, v1 + n);
            graph.AddEdge(v1, v2 + n);
            transgraph.AddEdge(v1 + n, v2);
            transgraph.AddEdge(v2 + n, v1);
        }
    }
    vector<Vertex> topologysort(2 * n);
    vector<bool> visited(graph.size(), false);
    for (size_t i = 0; i < graph.size(); ++i) {
        if (!visited[i]) {
            graph.ConnDFS(i, visited, topologysort);
        }
    }
    std::reverse(topologysort.begin(), topologysort.end());
    int colour = 0;
    vector<int> visited2(graph.size(), false);
    for (size_t i = 0; i < topologysort.size(); ++i) {
        if (!visited2[topologysort[i]]) {
            ++colour;
            transgraph.ColourDFS(topologysort[i], visited2, colour);
        }
    }
    for (int i = 0; i < n; ++i) {
        if (visited2[i] == visited2[i + n]) {
            cout << "Impossible";
            return 0;
        }
    }
    for (int i = 0; i < n; ++i) {
        if (visited2[i] < visited2[i+n]){
            cout << colours[i].first;
        } else {
            cout << colours[i].second;
        }
    }
    return 0;
}