LeetCode 141. Linked List Cycle

原题链接在这里：https://leetcode.com/problems/linked-list-cycle/

题目：

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

Follow up:
Can you solve it without using extra space?

题解：

快慢指针，一快runner, 每次走两步. 一慢walker, 每次走一步，若是它俩相遇就是有cycle.

Time Complexity: O(n). Space: O(1).

AC Java:

/**
 * 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 walker = head;
        ListNode runner = head;
        while(runner.next != null && runner.next.next != null){
            walker = walker.next;
            runner = runner.next.next;
            if(walker == runner){
                return true;
            }
        }
        return false;
    }
}

AC JavaScript:

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
    if(!head || !head.next){
        return false;
    }
    
    var walker = head;
    var runner = head;
    while(runner && runner.next){
        walker = walker.next;
        runner = runner.next.next;
        if(walker === runner){
            return true;
        }
    }
    
    return false;
};

跟上Linked List Cycle II, Circular Array Loop.