将两个升序链表合并为一个新的升序链表并返回。新链表是通过拼接给定的两个链表的所有节点组成的。 

示例：

输入：1->2->4, 1->3->4
输出：1->1->2->3->4->4

/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
  var mergedHead = { val : -1, next : null },
      crt = mergedHead;
  if (l1 === null) return l2;
  if (l2 === null) return l1;
  while(l1 && l2) {
    if (l1.val > l2.val) {
      crt.next = l2
      l2 = l2.next  // 指针2后移
    } else {
      crt.next = l1
      l1 = l1.next
    }
    crt = crt.next
  }
  crt.next = l1 || l2
  return mergedHead.next
};

/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
  if (l1 === null) return l2;
  if (l2 === null) return l1;
  if (l1.val < l2.val) {
    l1.next = mergeTwoLists(l1.next, l2);
    return l1;
  } else {
    l2.next = mergeTwoLists(l1, l2.next);
    return l2;
  }
};

var mergeTwoLists = function (l1, l2) {
  const prehead = new ListNode(-1);

  let prev = prehead;
  while (l1 != null && l2 != null) {
    if (l1.val <= l2.val) {
      prev.next = l1;
      l1 = l1.next;
    } else {
      prev.next = l2;
      l2 = l2.next;
    }
    prev = prev.next;
  }
  prev.next = l1 === null ? l2 : l1;
 /prev.next = l1 || l2;

  return prehead.next;
};