//classic linked list approach: use two pointers, one slow, one fast!
//After finding the cycle, move slow pointer to the head of LinkedList
//Then move the two pointers one step further at each time,
//when they meet again, the meeting point is the head node of cycle
public static Node FindLoop(Node head)
{
Node slow = head;
Node fast = head;
while (slow != null && fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast) //if two pointers meet, there is a loop
break;
}
//two cases: fast is now null or fast is at the last node of list
//so we need to check both fast and fast.next
//if either fast or fast.next is null, we know there is no cycle
//cannot check fast.next first, because if fast is now null, there
//gonna be nullpointer exception
if (fast == null || fast.next == null)
return null;
slow = head;
while (slow != fast) {
slow = slow.next;
fast = fast.next;
}
return slow;
}
//classic linked list approach:
//use two pointers, one slow, one fast!
public static bool FindLoop(Node head)
{
Node slow = head;
Node fast = head;
while (slow != null && fast != null && fast.next != null) {
slow = slow.next;
fast = fast.next.next;
if (slow == fast) //if two pointers meet, there is a loop
return true;
}
return false;
}
public static void Main (string[] args)
{
Node node1 = new Node (1);
Node node2 = new Node (2);
Node node3 = new Node (3);
Node node4 = new Node (4);
Node node5 = new Node (5);
Node node6 = new Node (6);
node1.next = node2;
node2.next = node3;
node3.next = node4;
node4.next = node5;
node5.next = node6;
node6.next = node4;
Console.WriteLine (Finder.FindLoop (node1).value);
}
public Node(int value)
{
this.value = value;
this.next = null;
}
