环形链表II

LeetCode-142

var detectCycle = function(head) {
  let fast = head
  let slow = head
  while(fast && fast.next) {
    fast = fast.next.next
    slow = slow.next
    if(fast === slow) break // 找到相遇节点
  }
  // 如果快指针(或其next)为null，则return null
  if(fast === null || fast.next === null) return null

  // 慢指针回到头节点
  slow = head
  while(slow !== fast) {
    // 快慢指针同步前进，相交点就是环起点
    slow = slow.next
    fast = fast.next
  }

  return fast // 返回fast或者slow
}