检查图中是否存在长度为3的环（满足给定条件）

#include <iostream>
#include <vector>

using namespace std;

bool checkCycle(vector<vector<int>>& graph, int v, vector<bool>& visited, int parent, int condition) {
    visited[v] = true;

    for (int u : graph[v]) {
        if (!visited[u]) {
            visited[u] = true;

            for (int w : graph[u]) {
                if (visited[w] && w != parent && condition == graph[v][u] + graph[u][w]) {
                    return true;
                }
            }

            visited[u] = false;
        }
    }

    return false;
}

bool hasCycleOfLength3(vector<vector<int>>& graph, int condition) {
    int numVertices = graph.size();
    vector<bool> visited(numVertices, false);

    for (int v = 0; v < numVertices; v++) {
        visited[v] = true;

        for (int u : graph[v]) {
            if (checkCycle(graph, u, visited, v, condition)) {
                return true;
            }
        }

        visited[v] = false;
    }

    return false;
}

int main() {
    int numVertices, numEdges;
    cout << "Enter the number of vertices and edges: ";
    cin >> numVertices >> numEdges;

    vector<vector<int>> graph(numVertices);

    cout << "Enter the connections between vertices (u, v) and their corresponding weights: " << endl;
    for (int i = 0; i < numEdges; i++) {
        int u, v, weight;
        cin >> u >> v >> weight;

        graph[u].push_back(v);
        graph[v].push_back(u);

        // Store the weight/condition between u and v
        graph[u][v] = weight;
        graph[v][u] = weight;
    }

    int condition;
    cout << "Enter the condition to be satisfied: ";
    cin >> condition;

    if (hasCycleOfLength3(graph, condition)) {
        cout << "Cycle of length 3 satisfying the condition exists." << endl;
    } else {
        cout << "Cycle of length 3 satisfying the condition does not exist." << endl;
    }

    return 0;
}

#include <iostream>
#include <vector>
#include <unordered_set>

using namespace std;

bool hasCycleOfLength3(vector<vector<int>>& graph) {
    int n = graph.size();
    
    for (int u = 0; u < n; ++u) {
        unordered_set<int> adjSet(graph[u].begin(), graph[u].end());

        for (int v : graph[u]) {
            for (int w : graph[v]) {
                if (w != u && adjSet.count(w) > 0) {
                    return true; // Cycle of length 3 found
                }
            }
        }
    }

    return false; // No cycle of length 3 found
}

int main() {
    // Create the graph as an adjacency list
    vector<vector<int>> graph = {
        {1, 2},
        {0, 2},
        {0, 1, 3},
        {2, 4},
        {3}
    };

    // Check if a cycle of length 3 exists
    bool cycleExists = hasCycleOfLength3(graph);

    // Print the result
    if (cycleExists) {
        cout << "A cycle of length 3 exists in the graph." << endl;
    } else {
        cout << "No cycle of length 3 exists in the graph." << endl;
    }

    return 0;
}