21. 合并两个有序链表

// 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。

function ListNode(val, next) {
  this.val = val === undefined ? 0 : val;
  this.next = next === undefined ? null : next;
}
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function(list1, list2) {
  if (!list1 && !list2) return null;
  const head = new ListNode();
  let p = head;
  while (list1 && list2) {
    if (list1.val < list2.val) {
      p.next = list1;
      list1 = list1.next;
    } else {
      p.next = list2;
      list2 = list2.next;
    }
    p = p.next;
  }

  if (list1) p.next = list1;
  if (list2) p.next = list2;

  return head.next;
};

console.log(
  JSON.stringify(
    mergeTwoLists(
      {
        val: 1,
        next: {
          val: 2,
          next: {
            val: 4,
            next: null,
          },
        },
      },
      {
        val: 1,
        next: {
          val: 3,
          next: {
            val: 4,
            next: null,
          },
        },
      }
    )
  )
); // [1,1,2,3,4,4]
console.log(mergeTwoLists(null, null)); // []
console.log(mergeTwoLists(null, { val: 0, next: null })); // [0]