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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/**
 * 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 || !head->next)    return false;

        ListNode *slow = head, *fast = head;

        while(fast->next && fast->next->next){
            fast = fast->next->next;
            slow = slow->next;

            if(slow == fast)    return true;
        }
        return false;
    }
};

142. Linked List Cycle II

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

/**
 * 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 *slow, *fast;
        slow = fast = head;

        while(fast != NULL && fast->next != NULL){
            fast = fast->next->next;
            slow = slow->next;
            if(slow == fast)    break;
        }

        if(slow != fast)    return NULL;
        slow = head;
        while(slow != fast){
            slow = slow->next;
            fast = fast->next;
        }

        return slow;
    }
};