142. Linked List Cycle II

找到环形链表的第一个节点

• 时间复杂度O(n)，空间复杂度O(1)
• Runtime: 88 ms, faster than 60.79%
• Memory Usage: 41.5 MB, less than 69.61%

快慢指针，快指针到达节点后，慢指针指向头节点，然后一起走，相等的节点就是环形链表的第一个交点。

/**
 * 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 || !head.next) return null;
  let slow = head.next;
  let fast = head.next.next;
  while (slow !== fast) {
    if (!fast || !fast.next) return null;
    slow = slow.next;
    fast = fast.next.next;
  }
  slow = head;
  while (slow !== fast) {
    slow = slow.next;
    fast = fast.next;
  }
  return slow;
};