142.leetcode Linked List Cycle II(medium)[有环链表]

七仔xu

于 2016-08-22 21:36:21 发布

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分类专栏： C++学习

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本文介绍了一种高效算法来检测链表中循环的起始节点。通过快慢指针法，无需额外空间即可找到循环开始的位置。文章还提供了一个C++实现示例。

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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.

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

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/**
 * 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) return NULL;
        ListNode *p = head;
        ListNode *q = head;
        if(p->next == NULL ||p->next->next == NULL) return NULL;
        p = head->next;
        q = p->next;
        while(1)
        {
            if(p == q) break;
            if(p->next == NULL) return NULL;
            p = p->next;
            if(q->next == NULL) return NULL;
            q = q->next;
            if(q->next == NULL) return  NULL;
            q = q->next;
            
        }
        ListNode *r = head;
        while(r->next != NULL && p->next != NULL && p != r)
        {
            r = r->next;
            p = p->next;
        }
        //cout<<r->val<<endl;
        return r;
    }
};