LeetCode_142_Linked_List_cycle_2.cpp

/*
  https://leetcode.com/problems/linked-list-cycle-ii/

  142. Linked List Cycle II
  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.
 */

/**
 * 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 == NULL ) {
      return NULL;
    }
    ListNode *slow = head;
    ListNode *fast = head;
    while( slow != NULL && fast != NULL ) {
      slow = slow->next;
      if( fast->next == NULL ) return NULL;
      fast = fast->next->next;
      if( slow == fast ) {
	// cycle detected
	int pos = 0;
	fast = head;
	while( fast != slow ) {
	  pos++;
	  fast = fast->next;
	  slow = slow->next;
	}
	return fast;
      }
    }
    return NULL;
  }
};