1721. 交换链表中的节点 #

var swapNodes = function(head, k) {
  let slow = fast = head
  while (k > 1) {
    fast = fast.next
    k--
  }
  const p1 = fast
  while(fast.next) {
    fast = fast.next
    slow = slow.next
  }
  const temp = p1.val
  p1.val = slow.val
  slow.val = temp
  return head
};