Linked List Cycle II

Posted 2020-08-12 wxquare的学习笔记

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

Note: Do not modify the linked list.

/**
 * 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* slow = head;
        ListNode* fast = head;
        while(fast && fast->next){
            slow = slow->next;
            fast = fast->next->next;
            if(slow == fast){
                ListNode* slow2 = head;
                while(slow2 != slow){
                    slow = slow->next;
                    slow2 = slow2->next; 
                }
                return slow;
            }
        }
        return nullptr;
    }
};