一.Linked List Cycle

Total Accepted: 85115 Total Submissions: 232388 Difficulty: Medium

 

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

Follow up:

Can you solve it without using extra space?

 

 

/** * 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 *first =head;        ListNode *second = head;        while(first && second)        {            first = first->next;            second = second->next;            if(second){                second = second->next;            }            if(first==second){                return true;            }        }        return false;    }};

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二.Linked List Cycle II

Total Accepted: 61662 Total Submissions: 196038 Difficulty: Medium

 

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?

 

 

 

/** * 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) {        ListNode* slow=head,*fast=head;        bool has_cycle = false;        while(slow && fast){            slow = slow->next;            fast = fast->next;            if(fast){                fast=fast->next;            }            if(slow == fast && slow){                has_cycle = true;                break;            }        }        ListNode* p = has_cycle ? head:NULL;        while(has_cycle && p!=slow){            p = p->next;            slow = slow->next;        }        return p;    }};

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