合并两个排序的链表

public class Solution {
    public ListNode Merge(ListNode list1,ListNode list2) {
        if(list1 == null && list2 == null) return null;
        else{
            ListNode list3 = new ListNode(0);  //先创建一个表头，方便往后加
            ListNode tem = list3;
            while(list1 != null && list2 != null){
                if(list1.val <= list2.val){
                    tem.next = list1;   //应该是把后边的全接过来了，但在后续操作中会断开，数据不会丢失。（不用新建结点赋值）
                    list1 = list1.next;
                    tem = tem.next;
                }
                else {
                    tem.next = list2;
                    list2 = list2.next;
                    tem = tem.next;
                }
            }
            if(list1 == null && list2 != null){//把剩余的加上去
                tem.next = list2;
            }
            else if(list2 == null && list1 != null){
                tem.next = list1;
            }

            return list3.next;
        }
    }
}

另 递归方法：

public ListNode Merge(ListNode list1,ListNode list2) {

       if(list1 == null){

           return list2;

       }

       if(list2 == null){

           return list1;

       }

       if(list1.val <= list2.val){

           list1.next = Merge(list1.next, list2);

           return list1;

       }else{

           list2.next = Merge(list1, list2.next);

           return list2;

       }       

   }