[Linked List]Linked List Cycle,Linked List Cycle II

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?

 (M) Linked List Cycle II

 

/**
 * 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;
    }
};

Next challenges: (M) Linked List Cycle II

 

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?

 (M) Linked List Cycle (H) Find the Duplicate Number

 

/**
 * 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;
    }
};

Next challenges: (H) Substring with Concatenation of All Words (E) Remove Linked List Elements (E) Delete Node in a Linked List

 

 

参考：

http://blog.sina.com.cn/s/blog_6f611c300101fs1l.html