https://medium.com/@Sazidz/how-to-detect-cycles-in-a-graph-6236e6eb750

2.cpp

void dfs(char node) {
    status[node] = VISITING;

    for (char child : graph[node]) {
        if (status[child] == UNVISITED)
            dfs(child);
        else if (status[child] == VISITING) {
            hasCycles = true;
            return;
        }
    }

    status[node] = VISITED;
}