141. Linked List Cycle

LinkedLists Greedy CycleDetection

Problem

Solution

Hash Set - O(n) time - O(n) space

class Solution {
public:
    bool hasCycle(ListNode *head) {
        unordered_set<ListNode*> list;
        while(head) {
            if (list.count(head)) return true;
            else {
                list.insert(head);
                head = head->next;
            }
        }
        return false;
    }
};

Two Pointer Floyd’s Cycle Detection- O(n) time - O(1) space

class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast && fast->next) {
            slow = slow->next;
            fast = fast->next->next;
            if (slow == fast) return true;
        }
        return false;
    }
};