【力扣刷题】21. 合并两个有序链表

输入：l1 = [1,2,4], l2 = [1,3,4]
输出：[1,1,2,3,4,4]

输入：l1 = [], l2 = []
输出：[]

输入：l1 = [], l2 = [0]
输出：[0]

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
       //新建一个头节点
        ListNode res = new ListNode(-101);
        //当前节点指针
        ListNode cur = res;
        List<Integer> list = new ArrayList<>();
        while (list1 != null || list2 != null) {
            if (list1 != null) {
                list.add(list1.val);
                list1 = list1.next;
            }
            if (list2 != null) {
                list.add(list2.val);
                list2 = list2.next;
            }
        }
        List<Integer> collect = list.stream().sorted().collect(Collectors.toList());
        for (int i = 0; i < collect.size(); i++) {
            ListNode listNode = new ListNode(collect.get(i));
            cur.next = listNode;
            cur = cur.next;
        }

        return res.next;
    }
}