LeetCode之Linked List Cycle II

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

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

之前的the beginning of the loop也有介紹

/**
 * 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) {
       ListNode *p = head, *q = head;
		 while (q && q->next){
			 p = p->next;
			 q = q->next->next;
			 if (q == p)
				 break;
		 }
		 if(q && q->next){
		     q = head;
		    while (p != q){
			     p = p->next;
			    q = q->next;
		    }
		    return p; 
        }
        return NULL;
    
    }
};