141. Linked List Cycle

Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

• Time : O(n)
  • if no cycle we can solve it O(n/2) is O(n)
  • if has cycle, there is cycle length (less then n) and two pointer’s distance is getting closer (-1 each move) so max time complexity is O(n) when cycle is in head.
• 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) {
        ListNode* one = head, *two = head;
        while(two and two->next and two->next) {
            one = one->next;
            two = two->next->next;
            if(one == two) return true;
        }
        return false;
    }
};