leetCode 142. 环形链表 II

参考:

代码随想录

leetCode 142. 环形链表 II_第1张图片

代码:

/**
 * 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 ||
         head->next->next == NULL)
        return NULL;
        ListNode* fast, *slow;
        fast = head;slow = head;
        fast = fast->next->next;
            slow = slow->next;
        while(fast != slow){
                //这里要先判断再后移 例如三个节点的单链表就会出错
            if(fast == NULL || fast->next == NULL){
                return NULL;
            }
            fast = fast->next->next;
            slow = slow->next;
        }
        ListNode* index = head;
        while(index != fast){
            index = index->next;
            fast = fast->next;
        }
        return index;
    }
};

 

141. 环形链表  (阉割版)

leetCode 142. 环形链表 II_第2张图片

 

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode* fast, *slow;
        if(!head||!head->next||!head->next->next)
        return false;
        fast = head->next->next;slow = head->next;
        while(fast != slow){
            if(!fast->next||!fast->next->next)
            return false;
            fast = fast->next->next;
            slow = slow->next;
        }
        return true;
    }
};

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