find_cycle_dfs.cpp

// Holds the graph
map<char, vector<char>> graph;

// Status of each node - UNVISITED, VISITING, VISITED
map<char, int> status;

// This will denote whether the graph has cycles
bool hasCycles = false;

// Status codes
const int UNVISITED = 0;
const int VISITING = 1;
const int VISITED = 2;

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;
}

int main() {
    // Input graph here
    
    for (char c : {'A', 'B', 'C'})
        if (status[c] == UNVISITED)
            dfs(c);

    if (hasCycles) {
        // Do something
    }
    
    return 0;
}