Solution.java

package linkedlistcycle;

/**
 * Given a linked list, determine if it has a cycle in it.
 *
 * To represent a cycle in the given linked list, we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to. If pos is -1, then there is no cycle in the linked list.
 *
 *
 *
 * Example 1:
 *
 * Input: head = [3,2,0,-4], pos = 1
 * Output: true
 * Explanation: There is a cycle in the linked list, where tail connects to the second node.
 *
 *
 * Example 2:
 *
 * Input: head = [1,2], pos = 0
 * Output: true
 * Explanation: There is a cycle in the linked list, where tail connects to the first node.
 *
 *
 * Example 3:
 *
 * Input: head = [1], pos = -1
 * Output: false
 * Explanation: There is no cycle in the linked list.
 *
 *
 *
 *
 * Follow up:
 *
 * Can you solve it using O(1) (i.e. constant) memory?
 */

class ListNode {
    int val;
    ListNode next;
    ListNode(int x) { val = x; }
}

class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode slow = head, fast = head;
        while(slow != null && fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast)
                return true;
        }
        return false;

    }
}

/*
Success
Details
Runtime: 0 ms, faster than 100.00% of Java online submissions for Linked List Cycle.
Memory Usage: 41.5 MB, less than 5.71% of Java online submissions for Linked List Cycle.
 */