Linked List Cycle

C++/6_Linked List Cycle.cpp

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+// C++ program to detect loop in a linked list
+//Floyd’s Cycle-Finding Algorithm 
+
+#include<iostream>
+#include <bits/stdc++.h>
+using namespace std;
+
+/* Link list node */
+typedef struct Node {
+	int data;
+	struct Node* next;
+}ListNode;
+
+void push(ListNode** head_ref, int new_data)
+{
+	/* allocate node */
+	ListNode* 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;
+}
+
+class Solution {
+public:
+    bool hasCycle(ListNode *head) {
+
+        //Take two pointers slow and fast
+
+        ListNode *current_pointer=head;
+
+        ListNode *ahead_pointer=head;
+
+    //Our slow pointer moves with us and fast pointer moves fast to check for loop or null address
+
+    //Our loop runs untill current,ahead and ahead->head which may mark the loop, are not null
+
+        while(current_pointer && ahead_pointer && ahead_pointer->next){
+
+            current_pointer=current_pointer->next; // we move current pointer in 1 step
+
+            ahead_pointer=ahead_pointer->next->next; // we move ahead pointer in two steps
+
+            if(current_pointer==ahead_pointer) return true; // loop is detected when both point to the same address
+
+        }
+        // in this algorithm ahead pointer moves over the same addresses for few times to detect the loop since current pointer is just at 1 step movement
+
+        return false;// when loop ends and reaches till here it means there might be null address encountered and no loop
+    }
+};
+// Returns true if there is a loop in linked list
+// else returns false.
+
+/* Main program to test our function*/
+int main()
+{
+	/* Start with the empty linked list */
+	ListNode* head = NULL;
+	int data;
+	//"Enter some data to continue and 0 to stop";
+	cin>>data;
+	while(data!=0){
+	    push(&head, data);
+	    cin>>data;
+	}
+
+	/* Create a loop for testing  so try to give input atleast with 4 nodes else loop can't be created*/
+	head->next->next->next->next = head;
+	Solution *ob;
+
+	if (ob->hasCycle(head))
+		cout << "Loop found";
+	else
+		cout << "No Loop";
+
+	return 0;
+}
+
+/*
+Ex-1: 
+
+3->2->0->4->2
+
+here loop is from 4 to 2 back
+
+current and ahead mark to head i.e;3
+Now,
+in loop 
+current(not null) and ahead(not null) and ahead->next(2)(not null)
+current=2
+ahead=0
+
+current(not null) and ahead(not null) and ahead->next(4)(not null)
+current=0
+ahead=2
+
+current(not null) and ahead(not null) and ahead->next(0)(not null)
+current=4
+ahead=4
+
+here,
+current==ahead =>loop is detected.Return true
+
+Ex-2: 
+
+1->2->3->4->5->2
+
+here loop is from 5 to 2 back
+
+current and ahead mark to head i.e;1
+Now,
+in loop 
+current(not null) and ahead(not null) and ahead->next(2)(not null)
+current=2
+ahead=3
+
+current(not null) and ahead(not null) and ahead->next(4)(not null)
+current=3
+ahead=5
+
+current(not null) and ahead(not null) and ahead->next(2)(not null)
+current=4
+ahead=3
+
+current(not null) and ahead(not null) and ahead->next(4)(not null)
+current=5
+ahead=5
+
+here,
+current==ahead =>loop is detected.Return true
+
+**Assume above input as addresses with data as the given numbers
+*/
+//Time Complexity : O(n) where n can be the length of the loop made from the head pointer
+//Space Complexity : O(1)