141. 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?

• use hash map to save node.

• Two pointers with different speed.

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head == null || head.next == null) return false;
        ListNode p = head, q = head.next;
        while(p != null && q != null){
            if(p == q) return true;
            p = p.next;
            q = q.next;
            if(q != null) q = q.next;
        }
        return false;
    }
}