合并两个排序的链表

public class Solution {
    // 非递归
    public ListNode Merge2(ListNode list1,ListNode list2) {
 
        ListNode dummy = new ListNode(0);
        ListNode lastNode = dummy;
         
        while(list1!=null && list2!=null){
            if(list1.val<list2.val){
                lastNode.next = list1;
                list1 = list1.next;
            }else{
                lastNode.next = list2;
                list2 = list2.next;
            }
             
            lastNode = lastNode.next;
        }
         
        if(list1!=null){
            lastNode.next = list1;
        }else{
            lastNode.next = list2;
        }
         
        return dummy.next;
    }
     
    //递归
    public ListNode Merge(ListNode list1,ListNode list2) {
        if(list1==null)
            return list2;
        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;
    }
}