0141-linked-list-cycle.cpp

/*
    Given head of a linked list, determine if it has a cycle in it

    Slow/fast pointers, if they ever intersect then there's a cycle

    Time: O(n)
    Space: O(1)
*/

/**
 * 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) {
            return false;
        }
        
        ListNode* slow = head;
        ListNode* fast = head;
        
        while (fast->next != NULL && fast->next->next != NULL) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) {
                return true;
            }
        }
        
        return false;
    }
};