代码的鲁棒性：合并两个排序的链表

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if (list1 == null) {
            return list2;
        } else if (list2 == null) {
            return list1;
        }
        ListNode mergeHead = null;
        if (list1.val < list2.val) {
            mergeHead = list1;
            mergeHead.next = Merge(list1.next, list2);
        } else {
            mergeHead = list2;
            mergeHead.next = Merge(list1, list2.next);
        }
        return mergeHead;        
    }
}