解题思路：

class Solution {public ListNode mergeTwoLists(ListNode list1, ListNode list2) {ListNode dum = new ListNode(0), cur = dum;while (list1 != null && list2 != null) {if (list1.val <= list2.val) {cur.next = list1;list1 = list1.next;} else {cur.next = list2;list2 = list2.next;}cur = cur.next;}cur.next = list1 != null ? list1 : list2;return dum.next;}
}