[21] 合并两个有序链表

/**
 * Definition for singly-linked list.
 * 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) {
  // 判断边界条件  
  if (!list1) {
    return list2;
  }
  if (!list2) {
    return list1;
  }

  // 此时l1 list2 都不为空  
  let dummyHead = new ListNode(-1);
  let last = dummyHead;

  while (list1 && list2) {
    if (list1.val < list2.val) {
      last.next = list1;
      last = list1;
      list1 = list1.next;
    } else {
      last.next = list2;
      last = list2;
      list2 = list2.next;
    }
  }

  // 此时说明l1或者l2为空  
  if (!list1) {
    last.next = list2;
  }
  if (!list2) {
    last.next = list1;
  }

  return dummyHead.next;
};