21. 合并两个有序链表

思路：

感觉没啥好说的，两链表每次取头指针进行判断，如果一方为空直接将另一方剩下的链表直接接到新链表上即可。如果两方都不为空就进行比较

代码：

class Solution {
        public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
            ListNode pre = new ListNode(0);
            ListNode curr = pre;
            while (l1 != null || l2 != null) {
                if (l1 == null) {
                    curr.next = l2;
                    return pre.next;
                }

                if (l2 == null) {
                    curr.next = l1;
                    return pre.next;
                }

                if (l1.val < l2.val) {
                    curr.next = l1;
                    curr = curr.next;
                    l1 = l1.next;
                } else {
                    curr.next = l2;
                    curr = curr.next;
                    l2 = l2.next;
                }
            }

            return pre.next;
        }
    }