leetcode21. 合并两个有序链表

 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class  {
    public static ListNode mergeTwoLists(ListNode l1, ListNode l2) {

        if (l1 == null && l2 == null) return null;

        ListNode result;
        if (l1 == null) {
            result = l2;
            l2 = l2.next;
        } else if (l2 == null) {
            result = l1;
            l1 = l1.next;
        } else if (l1.val >= l2.val) {
            result = l2;
            l2 = l2.next;
        } else {
            result = l1;
            l1 = l1.next;
        }

        ListNode head = result;

        while (!(l1 == null && l2 == null)) {
            System.out.println("aaa");
            if (l1 == null) {
                head.next = l2;
                l2 = null;
            } else if (l2 == null) {
                head.next = l1;
                l1 = null;
            } else if (l1.val >= l2.val) {
                head.next = l2;
                head = head.next;
                l2 = l2.next;
            } else {
                head.next = l1;
                head = head.next;
                l1 = l1.next;
            }
        }
        return result;
    }
}