142. Linked List Cycle II

Leetcode

https://leetcode.com/problems/linked-list-cycle-ii/

題目

Given the head of a linked list, return the node where the cycle begins. If there is no cycle, return null.

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 (0-indexed). It is -1 if there is no cycle. Note that pos is not passed as a parameter.

Do not modify the linked list.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
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: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.

Example 3:

Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.

解答

• 方法一

Floyd's Cycle Finding Algorithm

var detectCycle = function(head) {
    if(!head || !head.next) return null;
    
    // slow 最初位置
    let slow = head.next;
    // fast 最初位置
    let fast = head.next.next;
    while(slow !== fast) {
        if(fast === null || fast.next === null) return null;
        slow = slow.next;
        fast = fast.next.next;
    }

    // 把 slow 放回原位，然後兩方都各以一步同步前進，碰到時即為 cycle 的起點
    slow = head;
    while(slow !== fast) {
        slow = slow.next;
        fast = fast.next;
    }
    return slow;
};

Runtime: 88 ms, faster than 53.73% of JavaScript online submissions for Linked List Cycle II.

Memory Usage: 41.6 MB, less than 60.02% of JavaScript online submissions for Linked List Cycle II.