changes done

loopdetection.cpp

@@ -0,0 +1,69 @@
+// C++ program to detect loop in a linked list
+#include <bits/stdc++.h>
+using namespace std;
+
+/* Link list node */
+struct Node {
+    int data;
+    struct Node* next;
+};
+
+void push(struct Node** head_ref, int new_data)
+{
+    /* allocate node */
+    struct Node* new_node = new Node;
+
+    /* put in the data  */
+    new_node->data = new_data;
+
+    /* link the old list off the new node */
+    new_node->next = (*head_ref);
+
+    /* move the head to point to the new node */
+    (*head_ref) = new_node;
+}
+
+// Returns true if there is a loop in linked list
+// else returns false.
+bool detectLoop(struct Node* h)
+{
+    unordered_set<Node*> s;
+    while (h != NULL) {
+        // If this node is already present
+        // in hashmap it means there is a cycle
+        // (Because you will be encountering the
+        // node for the second time).
+        if (s.find(h) != s.end())
+            return true;
+
+        // If we are seeing the node for
+        // the first time, insert it in hash
+        s.insert(h);
+
+        h = h->next;
+    }
+
+    return false;
+}
+
+/* Driver program to test above function*/
+int main()
+{
+    /* Start with the empty list */
+    struct Node* head = NULL;
+
+    push(&head, 20);
+    push(&head, 4);
+    push(&head, 15);
+    push(&head, 10);
+
+    /* Create a loop for testing */
+    head->next->next->next->next = head;
+
+    if (detectLoop(head))
+        cout << "Loop found";
+    else
+        cout << "No Loop";
+
+    return 0;
+}