LeetCode-Linked List Cycle II

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于 2014-07-29 09:50:03 发布

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分类专栏： Cracking LeetCode HashTable List Circle Fast and Slow Pointers 文章标签： leetcode Linked List Cycle II

本文链接：https://blog.youkuaiyun.com/HolyDominat/article/details/38258437

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本文介绍了一种高效检测链表中循环开始节点的方法。通过使用快慢指针技巧，在不额外使用空间的情况下找到循环起点。文章包含完整的C++实现代码。

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

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

Solution:

Code:

<span style="font-size:14px;">/**
 * 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 *fast = head, *slow = head;
        while (fast != NULL && fast->next != NULL) {
            fast = fast->next->next;
            slow = slow->next;
            if (fast == slow) break;
        }
        if (fast != slow) return NULL;
        slow = head;
        while (fast != slow) {
            fast = fast->next;
            slow = slow->next;
        }
        return fast;
    }
};</span>