linkedListCycle.cpp

// Source : https://oj.leetcode.com/problems/linked-list-cycle/
// Author : Hao Chen
// Date   : 2014-07-03

/********************************************************************************** 
* 
* 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) {
        return hasCycle04(head);
        return hasCycle03(head);
        return hasCycle02(head);
        return hasCycle01(head);
    }


    // using a map to store the nodes we walked
    bool hasCycle01(ListNode *head) {
        unordered_map<ListNode*,bool > m;
        ListNode* p = head;
        m[(ListNode*)NULL] = true;
        while( m.find(p) == m.end() ) {
            m[p] = true;
            p = p -> next;
        }
        return p != NULL;
    }

    // Change the node's of value, mark the footprint by a special value
    bool hasCycle02(ListNode *head) {
        ListNode* p = head;
        // using INT_MAX as the mark could be a bug!
        while( p &&  p->val != INT_MAX ) {
            p->val = INT_MAX;
            p = p -> next;
        }
        return p != NULL;
    }

    /*
     * if there is a cycle in the list, then we can use two pointers travers the list.
     * one pointer traverse one step each time, another one traverse two steps each time.
     * so, those two pointers meet together, that means there must be a cycle inside the list.
     */
    bool hasCycle03(ListNode *head) {
        if (head==NULL || head->next==NULL) return false;
        ListNode* fast=head;
        ListNode* slow=head;
        do{
            slow = slow->next;
            fast = fast->next->next;
        }while(fast != NULL && fast->next != NULL && fast != slow);
        return fast == slow? true : false;
    }

    // broken all of nodes 
    bool hasCycle04(ListNode *head) {
         ListNode dummy (0);
        ListNode* p = NULL;

        while (head != NULL) {
            p = head;
            head = head->next;

            // Meet the old Node that next pointed to dummy
            //This is cycle of linked list
            if (p->next == &dummy) return true;

            p->next = &dummy; // next point to dummy
        }
        return false;
    }

};