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  • LC 417. Linked List Cycle II

    题目描述

    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.
    

    参考答案

    /**
     * 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 || head->next == NULL) return NULL;
            ListNode* fp = head;
            ListNode* sp = head;
            bool isCycle = false;
            
            while(fp != NULL && sp != NULL){
                fp = fp -> next;
                if(sp -> next == NULL) return NULL;
                sp = sp->next->next;
                if(fp == sp) {
                    isCycle = true;
                    break;
                }
            }
            
            if(!isCycle) return NULL;
            
            fp = head;
            while(fp != sp) {
                fp = fp->next;
                sp = sp->next;
            }
            return fp;
            
        }
    };

    答案注释

    想象 fast 的速度是一步,slow 的速度是不动。那么,常规情况下,他俩在a点集合,在a点再次相遇。

    但由于slow 摸鱼了,晚来了,fast已经走到b点了,slow才和fast集合,一起出发。a-b 就是所求的 循环开始点 到 列表开始点的距离。

    更详细解释,可参考:https://www.cnblogs.com/hiddenfox/p/3408931.html

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  • 原文地址:https://www.cnblogs.com/kykai/p/11625531.html
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