142.环形链表-ii.js

/*
 * @lc app=leetcode.cn id=142 lang=javascript
 *
 * [142] 环形链表 II
 */

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

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function(head) {
  if (head === null) return null
    let slow = head
    let fast = head

    while (fast.next !== null && fast.next.next !== null) {
        slow = slow.next;
        fast = fast.next.next;
        if (slow === fast) {
            fast = head;
            while (slow !== fast) { 
                slow = slow.next;
                fast = fast.next;
            }
            return fast
        };
    }

    return null
};
// @lc code=end