【剑指offer】合并两个排序的链表

public class  {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}

public ListNode Merge(ListNode list1, ListNode list2) {
    if (list1 == null)
        return list2;
    else if (list2 == null)
        return list1;

    ListNode head = null;

    if (list1.val < list2.val){
        head = list1;
        head.next = Merge(list1.next, list2);
    }
    else {
        head = list2;
        head.next = Merge(list1, list2.next);
    }

    return head;
}