From 27810c6695b1e6e40cb9aa4229388f4d4eff23b1 Mon Sep 17 00:00:00 2001
From: Nandini <132251059+Nanzz94@users.noreply.github.com>
Date: Sun, 6 Oct 2024 00:17:27 +0530
Subject: [PATCH] added Hamiltonian Cycle problem

Hamiltonian Cycle problem using a backtracking approach
---
 CP/HamiltonianCycle.cpp | 108 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 108 insertions(+)
 create mode 100644 CP/HamiltonianCycle.cpp

diff --git a/CP/HamiltonianCycle.cpp b/CP/HamiltonianCycle.cpp
new file mode 100644
index 0000000..5cac8e7
--- /dev/null
+++ b/CP/HamiltonianCycle.cpp
@@ -0,0 +1,108 @@
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+// Number of vertices in the graph
+#define N 5
+
+// Function to print the Hamiltonian Cycle
+void printCycle(const vector<int>& path) {
+    for (int vertex : path)
+        cout << vertex << " ";
+    // To show it's a cycle, print the first vertex again
+    cout << path[0] << endl;
+}
+
+// Utility function to check if the current vertex can be added to the Hamiltonian Cycle
+bool isSafe(int v, const vector<vector<int>>& graph, const vector<int>& path, int pos) {
+    // Check if the current vertex is adjacent to the previous vertex
+    if (graph[path[pos - 1]][v] == 0)
+        return false;
+
+    // Check if the current vertex has already been included in the path
+    for (int vertex : path)
+        if (vertex == v)
+            return false;
+
+    return true;
+}
+
+// Recursive function to solve the Hamiltonian Cycle problem
+bool hamCycleUtil(const vector<vector<int>>& graph, vector<int>& path, int pos) {
+    // Base case: If all vertices are included in the path
+    if (pos == N) {
+        // And if there is an edge from the last vertex to the first vertex
+        if (graph[path[pos - 1]][path[0]] == 1)
+            return true;
+        else
+            return false;
+    }
+
+    // Try different vertices as the next candidate in the Hamiltonian Cycle
+    for (int v = 1; v < N; v++) { // Starting from 1 since 0 is the starting vertex
+        if (isSafe(v, graph, path, pos)) {
+            path[pos] = v;
+
+            if (hamCycleUtil(graph, path, pos + 1))
+                return true;
+
+            // Backtrack if adding vertex v doesn't lead to a solution
+            path[pos] = -1;
+        }
+    }
+
+    // If no vertex can be added to the Hamiltonian Cycle
+    return false;
+}
+
+// Main function to solve the Hamiltonian Cycle problem
+bool hamCycle(const vector<vector<int>>& graph) {
+    vector<int> path(N, -1);
+
+    // Let us start at vertex 0
+    path[0] = 0;
+
+    if (!hamCycleUtil(graph, path, 1)) {
+        cout << "No Hamiltonian Cycle exists\n";
+        return false;
+    }
+
+    printCycle(path);
+    return true;
+}
+
+int main() {
+    /* Let us create the following graph
+        (0)--(1)--(2)
+         |   / \   |
+         |  /   \  |
+         | /     \ |
+        (3)-------(4)
+    */
+
+    vector<vector<int>> graph1 = {
+        {0, 1, 0, 1, 0},
+        {1, 0, 1, 1, 1},
+        {0, 1, 0, 0, 1},
+        {1, 1, 0, 0, 1},
+        {0, 1, 1, 1, 0}
+    };
+
+    cout << "Graph 1:\n";
+    hamCycle(graph1);
+
+    // Another graph without Hamiltonian Cycle
+    vector<vector<int>> graph2 = {
+        {0, 1, 0, 1, 0},
+        {1, 0, 1, 1, 1},
+        {0, 1, 0, 0, 0},
+        {1, 1, 0, 0, 1},
+        {0, 1, 0, 1, 0}
+    };
+
+    cout << "\nGraph 2:\n";
+    hamCycle(graph2);
+
+    return 0;
+}