142. Linked List Cycle II.cpp

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.

Note: Do not modify the linked list.



Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.


Example 2:

Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.


Example 3:

Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.




Follow-up:
Can you solve it without using extra space?











/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        if(!head) return NULL;
        if(head->next==NULL) return NULL;
        ListNode *slow=head, *fast=head;
        bool loop=false;
        while(fast!=NULL && fast->next!=NULL){
            slow=slow->next;
            fast=fast->next->next;
            if(slow==fast){
                loop=true;
                break;
            }
        }
        if(!loop) return NULL; // no cycle
        slow=head;
        while(slow!=fast && fast!=NULL){
            slow=slow->next;
            fast=fast->next;
        }
        return slow;
    }
};