19. 删除链表的倒数第 N 个结点

经典快慢双指针。

可参考下图，思路是一样的，细节可能不太一样。再注意边界问题就好

function ListNode(val, next) {
  this.val = (val === undefined ? 0 : val)
  this.next = (next === undefined ? null : next)
}
var removeNthFromEnd = function (head, n) {
  let slow = head, last = n, fast = head
  if (!head.next) return null
  while (last > 0) {
    fast = fast.next
    if (!fast) head = head.next
    last -= 1
  }
  while (fast && fast.next) {
    fast = fast.next
    slow = slow.next
  }
  slow.next = slow.next.next
  return head
};
// ----------------- test case -------------------
var l1 = new ListNode(1), l2 = new ListNode(2), l3 = new ListNode(3), l4 = new ListNode(4), l5 = new ListNode(5);
l1.next = l2
l2.next = l3
l3.next = l4
l4.next = l5
l5.next = null
var h1 = new ListNode(1)
var h2 = new ListNode(2)
var h3 = new ListNode(3)
h1.next = h2
h2.next = h3
const resultHead = removeNthFromEnd(h1, 3)