链表中与环相关的问题

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141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

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

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题目链接

利用一个快指针，一个慢指针。如果它们能相遇，则存在环。

/** * Language: cpp ( 9ms ) * 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) {        if(head==NULL||head->next==NULL){            return false;        }        ListNode* fast=head,*slow=head->next;        while(fast!=slow){            if(slow->next==NULL||slow->next->next==NULL){                return false;            }else{                fast=fast->next;                slow=slow->next->next;            }        }        return true;    }};

142. Linked List Cycle II

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?

题目链接

判断环的入口点：相遇点到环入口的距离等于头节点到环入口的距离。

class Solution {public:    ListNode *detectCycle(ListNode *head) {        if(head==NULL||head->next==NULL){            return NULL;        }        ListNode *slow=head,*fast=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;    }};

计算环的长度
如何判断两个链表（不带环）是否相交
资料参考

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