[leetcode 142] Linked List Cycle II

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

Notice that you should not modify the linked list.

 

사이클 시작 위치 노드를 찾아라!

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.

 

Constraints:

• The number of the nodes in the list is in the range [0, 104].
• -105 <= Node.val <= 105
• pos is -1 or a valid index in the linked-list.

문제 풀이:

연결리스트에서 cycle detection을 하는 문제이다. 지난번에 사용했던 토끼와 거북이 알고리즘을 사용하면 된다!!

토끼나 거북이 포인터를 출발지로 옮기지 않고 헤드를 그대로 옮기는 것도 가능하다.

/**
 * 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* hare = head;
        ListNode* tor = head;
        
        while(hare && hare->next){
            hare = hare->next->next;
            tor = tor->next;
            
            if(hare == tor){
                break;
            }
        }
        
        if(hare == NULL || hare->next == NULL){
            return nullptr;
        }
        
        while(head!=tor){
            head = head->next;
            tor = tor->next;
        }
        
        return head;
        
    }
};